Garage Conversion ADU in Boise
Transform your existing garage into a fully permitted accessory dwelling unit. The fastest, most affordable path to rental income, multigenerational living, or a private guest house in the Treasure Valley.
A garage conversion ADU transforms an existing attached or detached garage into a fully habitable, independently metered or sub-metered living space — complete with a kitchen or full kitchenette, a bathroom with shower, a sleeping area, a code-legal egress window, and a dedicated exterior entrance. Instead of building a new structure from the ground up, you repurpose a building that already has a slab, perimeter foundation, framed walls, and a weathertight roof. That single fact — reusing the structural shell — is what makes garage conversion the lowest-cost and fastest path to a legal accessory dwelling unit on a typical Boise residential lot.
In Idaho, an ADU is a smaller secondary dwelling on the same legal parcel as the primary residence. It has its own kitchen, its own bathroom, and its own entry, and it is permitted and inspected as a dwelling unit, not as an outbuilding or storage space. A garage that has been cleaned out, drywalled, and run with a window AC unit is not an ADU; it is unpermitted living space, and it creates real problems at resale, refinance, and insurance time. Every project we build is pulled as a permitted addition or conversion under the current adopted code, gets the required City of Boise or Ada County inspections, and ends with a certificate of occupancy that recognizes the new dwelling unit on the property record. Verify current code with your jurisdiction before you start, because ADU rules in the Treasure Valley have changed multiple times in recent years.
Garage conversions are the most affordable ADU option for most Boise homeowners because the structural shell already exists. The foundation slab, exterior walls, and roof assembly normally represent the most expensive phases of new ADU construction. By starting with a building that has all three of those phases already paid for, your budget goes directly into the elements that turn a parking structure into a comfortable home: a continuous insulation and air-sealing package that holds up to Treasure Valley winters, plumbing rough-in for the bathroom and kitchen, electrical service capacity adequate for a real dwelling, an HVAC strategy that does not lean on the main house, egress windows, the garage-door infill, and interior finishes that you will actually want to live in or rent out. A typical finished conversion yields 300 to 600 square feet of living area, depending on whether you start with a single-car, two-car, or oversized three-car / shop-style garage.
In the current Boise ADU market we see garage conversions chosen most often for three reasons: a homeowner wants rental income on the property and wants it online inside a year, a family is bringing in aging parents and needs an accessible single-level unit with no stairs, or someone wants a private guest suite or home-office casita without the six-to-twelve month timeline and significantly higher cost of a ground-up detached ADU. Conversions typically move from permit issuance to certificate of occupancy in 2 to 4 months of active construction, which is roughly half the schedule of a detached new-build of similar size. For homeowners on a financing timeline or trying to seat a tenant before a tax year ends, that schedule advantage is often as important as the cost difference.
Iron Crest Remodel (Iron Crest Remodeling Group LLC, Idaho RCE-6681702) has completed garage conversions across the Boise metro — from compact single-bay studios in the North End and East End, to full two-car conversions in Meridian, Eagle, Kuna, and Nampa, to oversized shop conversions on larger lots out toward Star and Middleton. Every project starts with a paid feasibility visit where we open inspection points on the slab, check the wall framing, verify ceiling height, photograph the existing electrical service, and shoot grades from the garage to the main sewer cleanout. That up-front diligence is the difference between a conversion that hits budget and one that becomes a stack of change orders three weeks in. If you want a broader view of every ADU type before going deeper here, our ADU construction overview compares conversion against new-build options side by side.
The economics of a garage conversion are straightforward: you already own the most expensive components of a building. Foundation, framing, exterior walls, and roof normally represent roughly 40 to 60 percent of new-construction cost. By converting an existing garage you bypass tens of thousands of dollars of structural work that every other ADU type requires. Here is exactly where those savings come from, and where they sometimes evaporate.
Existing Structure Eliminates Whole Phases
Your garage already has a poured slab, standing walls, and a weathertight roof. Those three elements are the largest single cost in any new construction project. A detached ADU of similar size requires excavation, footings, slab, framing, sheathing, roof trusses, roofing, and weather barrier before a single light switch is wired. With a conversion, your investment goes directly into making the interior livable — insulation, mechanical systems, plumbing, and finishes — rather than building the box. That is where the conversion savings advantage comes from on every project we quote.
Faster Timeline Means Lower Carrying Costs
Because the shell already exists, construction focuses entirely on interior buildout and mechanical systems. A typical Boise conversion moves from permit approval to certificate of occupancy in 8 to 16 weeks of active construction — roughly half the on-site schedule of a detached ADU. Faster completion lowers labor costs, reduces the time your driveway is a construction zone, and gets rental income or family occupancy online sooner. If you are financing the project, fewer months of interest carry can be worth several thousand dollars on its own.
No New Foundation Excavation
Your garage slab is already poured, cured, and (on most homes built since the late 1980s) sitting on perimeter footings that bear below Boise's nominal 24-inch frost depth. While the slab may still need a vapor barrier, rigid-foam insulation, or a structural topping in some cases, you avoid the full cost of excavation, formwork, rebar, concrete pump, pouring, and cure time. In Treasure Valley clay-and-cobble soils, new foundation work on a detached ADU is a meaningful budget line. A conversion eliminates almost all of it, provided the existing footings are adequate — which we verify, not assume.
Utilities Already Nearby
Most Boise garages already have at least lighting circuits and an opener circuit running back to the main panel, attached garages typically share a wall with the home's kitchen or laundry plumbing, and detached garages are usually within 20 to 60 feet of existing utility connections. That proximity routinely saves several thousand dollars in trenching and service-extension work versus building at the rear of the lot. Detached garages still need a real trench for sewer, water, and a sub-feed — we never pretend that is free — but the runs are short compared to a backyard cottage at the property line.
No New Roof Required (If The Existing One Is Good)
The existing roof assembly — trusses or rafters, sheathing, underlayment, and roofing material — stays in place. As long as the roof has meaningful service life left, you simply add code-required insulation to the ceiling plane or up against the roof deck. New roofs on detached ADUs are a real line item that conversions avoid. The honest caveat: if the garage roof is at end-of-life, you should re-roof now while everything is open, and that cost has to be added to the conversion budget rather than hidden. We inspect roof condition on every feasibility visit.
Where The Savings Can Evaporate
Conversion savings assume the existing structure is sound. If the slab has settled or cracked, if the perimeter footings do not bear below frost depth, if the walls are uninsulated single-wall framing that effectively has to be rebuilt, if the roof needs replacement, if there is no sewer lateral within reasonable trenching distance, or if the existing electrical service has no room for a new ADU subpanel, the "savings" of conversion shrink fast. On those properties, a clean-sheet detached ADU is sometimes a better financial decision than fighting a tired building. We will tell you which side of that line your project falls on before you commit.
Garage conversion cost in Boise depends on the size of the existing garage, the condition of the slab and shell, the distance and difficulty of the plumbing and electrical work, and your finish level. The ranges below reflect projects we have completed and bid in Ada and Canyon counties during 2025 and 2026. These are project ranges, not fixed prices — every property gets a written, itemized estimate after a feasibility visit.
| Garage Type | Sq Ft Range | Typical Installed Range |
|---|---|---|
| 1-Car Garage / Single-Bay Studio | 250 – 350 sq ft | $80,000 – $120,000 |
| 2-Car Garage / 1-Bed Unit | 400 – 600 sq ft | $100,000 – $150,000 |
| 3-Car / Shop-Style Garage | 550 – 750 sq ft | $120,000 – $180,000 |
| Partial (Half of 2-Car) Conversion | 200 – 300 sq ft | $65,000 – $95,000 |
Per square foot, Boise garage conversions typically land in the $250 to $350 range depending on scope, finish level, and how much hidden upgrade work the existing structure requires.
Where The Money Actually Goes
Insulation & Air Sealing (Zone 5)
$4,000 – $9,000
HVAC (Cold-Climate Mini-Split)
$4,000 – $8,000
Plumbing Rough-In (Bath + Kitchen)
$8,000 – $18,000
Sewer Trench & Tie-In (Detached)
$3,000 – $9,000
Electrical Subpanel & Wiring
$5,000 – $12,000
200A Main Service Upgrade (if needed)
$2,500 – $5,000
Slab Prep, Vapor Barrier, Subfloor
$2,000 – $7,000
Egress + Daylight Windows
$3,000 – $8,000
Entry Door + Garage-Door Infill
$3,000 – $8,000
Kitchen Cabinets & Counters
$6,000 – $15,000
Bath Fixtures, Tile, Shower Pan
$6,000 – $14,000
Permits, Plan Review, Inspections
$2,000 – $6,000
The single biggest cost variable on a Boise conversion is plumbing distance. An attached garage that shares a wall with the kitchen or main bath might tie in for under ten thousand dollars in total plumbing. A detached garage 60 feet from the cleanout, with a driveway in the way and an ejector pump required because the slab sits below the sewer main, can run double that. The second biggest variable is electrical service capacity — a home already at 200 amps with panel space costs much less to extend than a 100-amp home that needs a full service upgrade before the ADU subpanel can be added. We do the load calculation and a grade shot on every estimate so those numbers are real, not optimistic. For a side-by-side cost comparison against a backyard cottage, see our detached vs. garage conversion ADU comparison.
The savings story above is real, but only when the existing garage is structurally and mechanically ready to become a dwelling unit. The single most common reason a Boise garage conversion runs over budget is that someone priced it as if the existing structure were code-ready when it was not. Here are the items we look at on every feasibility visit before quoting hard numbers.
Slab Condition & Moisture
A typical residential garage slab is 4 inches of unreinforced concrete with no vapor barrier underneath. That is fine for a vehicle. For a dwelling, it is a wet floor waiting to happen. We test for moisture migration with a plastic-sheet test or a meter, look for hairline cracking and settlement at the apron, and check for any sign of slope toward the old overhead-door opening (because garages were intentionally sloped to drain out, which is the wrong direction for a finished floor). The fix is some combination of a continuous vapor barrier, rigid foam, a level engineered subfloor, and in some cases a structural topping slab. None of that is optional in a Treasure Valley winter.
Foundation & Frost-Depth Check
Habitable structures in the Boise area need footings bearing below the local frost depth, generally taken as 24 inches. Most attached garages share the main house footing and are fine. A non-trivial number of older detached garages, carport enclosures, and homeowner-built additions were built on a thickened slab edge with no real footing. We open at least one inspection pit at the perimeter to verify what is actually down there. If footings are inadequate, the remedy is localized underpinning or a new perimeter footing tied into the existing slab. Cost varies a lot by access, and we will quote it honestly rather than pretending it is not a line item.
Insulation & The Climate Zone 5 Envelope
Once you convert the space, it counts as conditioned space, and the entire shell has to meet the current energy code adopted by Idaho. In Boise (IECC Climate Zone 5) that typically means R-21 walls, R-49 ceiling/roof, R-10 slab edge or under-slab insulation, U-0.30 or better windows, and a blower-door air-tightness test. Existing garage walls are usually 2x4 framing with zero insulation. Solutions include furring out to 2x6 and using batt or dense-pack, or applying closed-cell spray foam to existing 2x4 cavities to hit code R-value without losing interior dimension. Verify current code with your jurisdiction, because Idaho updates the adopted code periodically. The envelope upgrade is non-negotiable, and it is one of the single largest line items in any honest conversion estimate.
Egress & Natural-Light Windows
Sleeping rooms need a code-sized emergency escape and rescue opening. Garages almost never have one. Habitable rooms also need natural light equivalent to roughly 8 percent of floor area and natural ventilation roughly 4 percent of floor area (or mechanical equivalent). That means cutting new windows into the existing walls, sometimes through concrete or CMU on older detached garages. We always design the egress location during schematic design rather than figuring it out in the field, because window placement drives furniture layout, exterior elevations, and street appearance.
Electrical Service Capacity
A real ADU draws real power: range or induction cooktop, heat pump, water heater, refrigerator, dryer, lighting, and outlets. The load calculation almost always lands between 60 and 100 amps for a dedicated subpanel. Many older Boise homes have 100-amp main service that is already most of the way full. The fix is a 200-amp main service upgrade plus a new ADU subpanel. We do the calc up front so the upgrade is in the contract, not a surprise on week three.
Plumbing Rough-In From The Main House
Attached garages that share a wall with the home's kitchen or main bathroom can often tie in for relatively short pipe runs. Detached garages require trenching for water (deep enough to stay below frost), sanitary drain (with correct fall to the sewer main), vent piping, and sometimes natural gas. If the garage slab sits below the elevation of the home's sewer lateral, gravity drainage will not work and we add a sealed ejector pump or up-flush system for the bathroom and kitchen waste. None of this is optional, and the grades have to be verified, not assumed.
Garage-Door Infill & Streetscape
Removing the overhead door is the single most visible change. Done badly it looks obviously like a closed-up garage; done well it looks like the house was always built that way. The infill includes a structural header to carry whatever load the old door track and lintel were carrying, full-depth framing tied into the surrounding walls, sheathing, weather-resistive barrier, insulation, and exterior cladding chosen to match the existing siding profile, exposure, and color. We pull paint and siding samples and rebuild the elevation so it reads as a wall, not a patch.
Ceiling Height After Finishes
Habitable rooms need 7 feet of finished ceiling height; bathrooms, hallways, and laundry can be 6 feet 8 inches. A standard 8-foot garage ceiling has only 12 inches of margin to absorb ceiling insulation, drywall, lighting, a new floor assembly, and any duct or piping that has to run horizontally. We model the ceiling assembly during design and choose strategies — recessed cans, ductless mini-splits instead of ducted air, shallow LED lighting — that keep finished heights legal.
Converting a garage into a real dwelling unit is more involved than finishing a basement. The following sections walk through how Iron Crest Remodel approaches each major system on a Treasure Valley conversion.
Foundation & Slab Upgrades
We evaluate the slab for thickness, cracking, settlement, moisture, and slope before any trades arrive. Standard remediation in Boise: clean and dry the slab, install a continuous vapor barrier, lay R-10 rigid foam, install a tongue-and-groove engineered subfloor, then build interior partitions on top. Where the slab is sloped toward the old overhead door, we either level with a self-leveling compound or build a sleeper system to bring the floor to plane. Where load capacity is questionable, we add a structural topping slab of 1.5 to 2 inches of fiber-reinforced concrete tied to the existing slab. Cracked or settled slabs get crack-stitching, polyurethane injection, or selective re-pour depending on what we find.
Garage-Door Opening Infill
The overhead door, springs, tracks, opener, and lintel come out. We size a new header to carry whatever load the lintel was carrying (often a header is already there and we are just verifying its capacity), then frame the full opening at the same wall depth as the surrounding wall — usually 2x6 to give us cavity space for code-level insulation. From the outside, we install matching weather-resistive barrier, flashing, and siding so the elevation reads as an original wall. From the inside, the choice is between a mostly solid wall with two well-placed double-hung windows and a new pedestrian entry, or a glazing-forward layout with a wall of windows or a sliding patio door to maximize light. Solid-wall infill is cheaper and gives more furniture wall; glass-forward looks better from inside but costs more and limits where you can place a sofa or a bed.
Ceiling, Roof, And Insulation Strategy
If the garage has an attic above the trusses, we typically blow R-49 cellulose or fiberglass at the ceiling plane and leave the attic vented. If the garage has a low-slope or cathedral roof with no attic, we go to closed-cell spray foam directly under the deck to hit R-value while preserving headroom. In both cases the air barrier is continuous and detailed at every penetration: can lights, exhaust fans, mini-split line-set, plumbing stack. The goal is a building that passes a blower-door test, not just one that has insulation in the walls.
HVAC: Why Mini-Splits Win In Boise
Cold-climate ductless mini-splits are the default HVAC choice on Boise garage conversions. A single-zone wall cassette handles heating down well below local design temperatures, cools all summer, runs off the new electrical subpanel, and avoids the headroom hit of ducted air in a low-ceiling space. Cost is reasonable, efficiency is excellent, and the unit is independent of the main house's system. For attached garages where the main home's ducted system has spare capacity, extending it can be an option, but only after a proper Manual J load calc on the upsized house — bolting an ADU onto an already-undersized system is how main-house comfort gets ruined.
Plumbing Path & Drain Strategy
We chase the shortest reasonable path from the new bathroom and kitchen back to the main sewer lateral, with the right fall, the right cleanouts, and proper venting. On attached garages this is usually inside conditioned space and straightforward. On detached garages we trench, sleeve water and gas, slope drain piping properly, and protect water from frost with depth and insulation. Where elevation does not allow gravity drainage, the ADU bathroom and kitchen run on a sealed ejector pump system sized for continuous residential use rather than the lighter-duty up-flush units sold for occasional half-baths. We do not improvise this; we shoot grades during design.
Electrical: Subpanel, Service, And Code-Required Devices
Every conversion gets a dedicated subpanel fed from the main service with appropriately sized conductors and overcurrent protection. We install AFCI/GFCI protection per current code, tamper-resistant receptacles, code-spacing on outlets, hardwired interconnected smoke and CO alarms throughout the unit, and dedicated circuits for the kitchen small-appliance loads, range, refrigerator, dishwasher, microwave, bath, laundry, and HVAC. Where the main service has to be upgraded to 200 amps, we coordinate the utility cut-over and inspection so the home is not without power overnight.
Fire Separation On Attached Garages
If the garage is attached to the main home, the shared wall already has a fire separation rating, typically 5/8-inch Type X gypsum on the garage side. When the garage becomes living space, the separation requirement changes depending on what is on each side and on the current adopted code. We confirm exactly what is required with the plan reviewer rather than guessing, and we maintain the continuity of any required rated assembly when new outlets, switches, recessed lights, or plumbing penetrate it. Hardwired interconnected smoke and CO detectors are installed in both the main home and the new unit per code.
The honest tradeoff of any garage conversion is parking. You are converting enclosed, weatherproof, sometimes-heated covered parking into something else. In a Treasure Valley winter with freezing fog, ice storms, and the occasional foot of snow on a windshield, covered parking is not nothing. Whether the conversion still pencils depends on the neighborhood, the comps, and what alternative parking you can offer.
In dense urban Boise — the North End, East End, Hyde Park, Central Bench — many homes already rely on street parking and a short driveway, and buyers in those neighborhoods are buying for walkability, mature trees, and original character, not for a 3-car garage. Converting a garage in those neighborhoods generally reads as adding livable area without breaking the streetscape, and the ADU rental income often outperforms the lost garage appraisal value. In suburban Meridian, Eagle, Star, Kuna, and Nampa, garages count for more on the comp sheet. Removing all covered parking on a $700K Eagle home can flatten resale value even after adding an ADU; that is why we so often recommend converting only one bay of a 2-car garage in those markets, or pairing the conversion with a new permitted carport so the MLS listing still says "covered parking."
Treat parking as a design decision, not an afterthought. Options we routinely build alongside a conversion include extending the driveway to add a tandem space, building a freestanding carport that matches the home's architectural style, adding a small detached storage shed to recapture the lost garage storage function, and reorganizing the driveway apron so guests, tenants, and homeowners are not blocking each other. The combined cost of those parking solutions is almost always a small fraction of the value the new ADU adds.
A garage conversion is not the right move for every property. Here is an honest assessment of the advantages and limitations based on Boise's building codes, Treasure Valley climate, and the current resale market.
Advantages
- Lowest installed cost per square foot of any ADU type, when the shell is sound
- Fastest construction schedule — typically 8 to 16 weeks on site
- Reuses existing foundation, framing, and roof — no new structural shell
- Often no change to lot coverage, setbacks, or stormwater calculations
- Streetscape stays mostly intact when the door infill is done well
- Easier financing in many cases — smaller loan amount than a detached new build
- Single-level layout is ideal for aging parents and accessibility needs
Limitations
- Loss of enclosed parking and the storage function that came with it
- Locked to the existing garage footprint — hard to add square footage
- Ceiling height after finishes is often the binding constraint
- Old slabs, thin walls, and end-of-life roofs can erase the cost savings
- Plumbing path on detached garages can be expensive and disruptive
- Hurts resale in higher-end suburban markets that expect a 2- or 3-car garage
- Existing electrical service may need a 200A upgrade before the ADU is feasible
Garage conversions are reviewed against the same building, energy, plumbing, mechanical, and electrical codes as any new residential living space, plus whatever ADU-specific zoning ordinance applies to your property. Inside the City of Boise the review is run by Planning and Development Services; outside city limits in unincorporated Ada County the review is run by Ada County Development Services. Rules differ. Always verify current code with the jurisdiction that actually controls your parcel before you sign a contract.
Zoning, Setbacks, And Lot Coverage
Most residential zones in the City of Boise allow at least one ADU on a single-family lot, subject to size caps, setback rules, and (in some cases) owner-occupancy requirements. Because the garage is already there, conversion typically does not trigger new setback or lot coverage calculations — that is one of the quiet advantages over new construction. But this is exactly the area where rules have changed over the past several legislative cycles. We confirm the current ADU ordinance, size cap, and any owner-occupancy clause with the jurisdiction in writing before finalizing your design.
IRC Habitable-Space Standards
Habitable rooms need a minimum of 70 square feet of floor area with no dimension less than 7 feet, natural light equal to about 8 percent of floor area, and natural ventilation equal to about 4 percent of floor area (or mechanical equivalent). Kitchens are exempt from the minimum area requirement but still have to meet light and ventilation rules. We design layouts on a grid that respects these minimums so nothing has to be moved during plan review.
Egress Windows
Every sleeping room needs an emergency escape and rescue opening with at least 5.7 square feet of net clear opening, 20 inches minimum width, 24 inches minimum height, and a sill no higher than 44 inches above the finished floor. Cutting a new egress opening in wood-framed garage walls is routine; in concrete or CMU walls (older detached garages, especially in the Central Bench and around Garden City), it requires a structural lintel and saw cutting, which adds cost.
Ceiling Height
7-foot finished minimum over at least 50 percent of the required floor area in habitable rooms; 6 feet 8 inches in bathrooms, hallways, and laundry. We measure existing height, model finish-layer assemblies, and confirm legal height before construction starts — not when the framing inspector is on site.
Energy Code — Climate Zone 5
Boise is in IECC Climate Zone 5. Once a garage becomes a dwelling unit, the entire envelope is treated as new conditioned space. Typical targets: R-21 walls, R-49 attic or roof, R-10 slab edge, U-0.30 windows, and a blower-door air-leakage test below the current adopted threshold. Walls in older garages routinely have to be furred out or spray-foamed to hit those numbers. Verify current code, because the adopted version in Idaho is updated periodically and the targets do shift.
Fire Separation (Attached Garages)
The wall between an attached garage and the rest of the house already carries a fire-separation requirement. When the garage becomes habitable space, the requirement on that wall changes depending on what occupancy is on each side. We coordinate the specifics with the plan reviewer at design time, maintain the continuity of any required rated assembly through new outlets and lights, and install hardwired interconnected smoke and CO alarms throughout the unit per current code.
The other ADU question we get more than any other is whether to convert the existing garage or build a new detached cottage in the backyard. The short answer: conversion wins on cost and schedule when the existing structure is sound; detached new-build wins on flexibility, long-term layout, and resale in higher-end markets. Here is the comparison we walk every homeowner through.
| Factor | Garage Conversion | Detached New Build |
|---|---|---|
| Typical Installed Cost | $80K – $180K | $200K – $400K+ |
| Active Construction Schedule | 8 – 16 weeks | 5 – 9 months |
| Typical Finished Size | 300 – 600 sq ft | 500 – 900+ sq ft |
| Layout Flexibility | Constrained by garage footprint | Design from scratch |
| Parking Impact | Loses covered parking | Keeps existing garage intact |
| Yard Impact | Minimal — no yard space lost | Takes a meaningful chunk of yard |
| Setback / Lot-Coverage Triggers | Usually none new | Full review |
| Resale In Higher-End Suburbs | Mixed — covered parking matters | Strong — adds livable area without removing garage |
For a deeper side-by-side, see our dedicated detached vs. garage conversion ADU comparison. If you want to see how the basement option stacks up, we cover that on our basement ADU page, and the above-garage second-story option is broken out on the above-garage ADU page.
The reason most homeowners convert a garage in the first place is income or family. Both math problems work in Boise right now, but the answers look very different.
For long-term rental, a clean, code-legal one-bedroom or studio ADU in the Boise metro currently commands meaningful monthly rent, and a smaller half-of-two-car studio rents for less but still produces a real return on a smaller construction budget. We will not quote a specific dollar figure here because Boise rents have moved a lot over the last several years and any number we print today could be stale tomorrow — pull current comps from property managers in your specific ZIP code before underwriting the project. What we can say is that the rent-to-build ratio on conversions tends to be the most favorable of any ADU type we build, because conversion gets to dwelling-unit status at the lowest installed cost.
For multigenerational living, the math is different. A single-level, no-stairs converted garage with a curbless shower, 36-inch interior doorways, lever hardware, and a wide kitchen aisle is one of the best aging-in-place layouts you can build on a Boise lot. The ROI in that case is not measured in monthly rent; it is measured against the cost of assisted living in the Treasure Valley. The conversion typically pays for itself in 12 to 36 months on that comparison while keeping the family on the same property.
Either way, the financing path matters. Homeowners typically use a cash-out refinance, a home equity line of credit, or a renovation-purpose loan. We are not lenders and we will not push a product, but we will give you a detailed line-item budget and a written milestone schedule that fit on a draw calendar, which is what every renovation lender asks for. If you want to see how the broader ADU project pencils on your specific parcel, the free in-home estimate request is the right next step.
Not every garage is an ideal conversion candidate. The following property types consistently produce the best cost efficiency, livability, and return on investment in the Treasure Valley market.
2-Car Garages With One Bay Converted
The most popular configuration we build in Boise. Convert one bay into a 200 to 300 square foot studio, keep the other bay for a vehicle, workshop, or storage. You preserve covered parking (which matters at resale in suburban markets), reduce construction cost, and still create a real rental or in-law unit. The dividing wall between bays is built to meet the current garage-to-dwelling separation requirements with continuous air, fire, and sound assemblies.
Detached Garages With Alley Access
Detached garages that face an alley or rear lane convert beautifully. The tenant has a separate entrance path without crossing the homeowner's yard, utility connections are usually short, and the two households get real privacy from each other. North End, Hyde Park, East End, and parts of the Central Bench have block patterns built for this configuration, and the resulting ADUs tend to lease quickly because the layout reads as a real cottage rather than a converted shed.
Oversized Garages In Older Neighborhoods
Mid-century Boise homes (1950s through 1970s) frequently have oversized garages — 24 by 28 feet or larger — that were originally built for workshop space, boat storage, or three vehicles. The extra footprint yields 500 to 700 square foot ADUs with full kitchens, a separated bedroom, and a real living area. Larger conversions of this type tend to outperform the rent-per-square-foot of compact studios because the unit competes with small apartments rather than with sleeping rooms.
Homes With Street Parking Or Carport Alternatives
Lots with ample street parking, a long driveway, or room for a freestanding carport absorb the loss of garage parking with the least resale pain. Many older urban Boise blocks already operate this way. Pair the conversion with a properly permitted carport matched to the home's architecture and the property still markets as a covered-parking home at resale, even though the original garage is now a dwelling unit.
We turn down conversion projects every year because the property is not a good fit, and we would rather lose the sale than build something that lets the homeowner down two years later. If any of the following apply to your property, look hard at an alternative ADU type before committing to conversion.
- The slab has settled or significantly cracked, or the perimeter footing does not bear below frost depth.
- The existing walls are uninsulated single-wall or board-on-batten construction that effectively has to be torn down and rebuilt.
- The roof is at end-of-life and a re-roof would have to be added to the conversion budget on top of everything else.
- Ceiling height is too low to absorb code-required insulation and flooring assemblies and still hit a 7-foot finished ceiling.
- Setbacks have changed since the garage was built and the existing structure is too close to the property line to legally house living space under current code.
- There is no reasonable plumbing path back to the main sewer lateral, or the slab elevation forces an ejector pump on a long, expensive trench.
- The home is in a neighborhood where every comparable home has a 2- or 3-car garage and removing all covered parking would visibly hurt resale.
- The owner wants a real two-bedroom unit with a separated living room, which often does not fit comfortably inside a single-car footprint.
- Zoning for the parcel does not currently allow an ADU, or already has another dwelling unit that caps the lot.
On those properties we usually point homeowners at a detached ADU, an attached ADU addition, a basement ADU, or an above-garage second-story unit, and we explain why on that specific lot.
Do I need a permit to convert my garage into an ADU in Boise?
Yes. A garage-to-ADU conversion inside Boise city limits requires a building permit from City of Boise Planning and Development Services, plus separate trade permits for plumbing, electrical, and mechanical scope. Outside city limits in unincorporated Ada County, the permit is issued by Ada County Development Services and may follow slightly different ADU rules. We always advise homeowners to verify current code and current ADU ordinances with the appropriate jurisdiction before signing a contract, because zoning, parking, and owner-occupancy rules in the Treasure Valley have changed multiple times over the last several years. Iron Crest Remodel prepares and submits the plan set, handles plan-check corrections, and coordinates every inspection so you do not chase paperwork.
How long does a garage conversion take in the Boise area?
Plan on roughly 4 to 6 months from contract signing to certificate of occupancy in most cases. Design and engineering typically take 3 to 5 weeks, plan review with the City of Boise or Ada County typically runs 4 to 8 weeks (longer during peak season), and active construction is usually 8 to 16 weeks depending on whether you are doing a single-bay studio or a full two-car one-bedroom unit. Detached garages where new plumbing has to be trenched from the main house add time. We give you a written milestone schedule at contract so you can plan tenants, family move-ins, or financing draws around real dates rather than guesses.
Will I lose too much property value by eliminating my garage in Boise's market?
In most Treasure Valley neighborhoods today, a permitted ADU adds more value than the enclosed garage was worth. A finished, code-legal ADU of 400 to 600 square feet typically appraises as additional living area on the tax record and adds meaningful rental income, while a garage appraises mostly as a covered-parking amenity. The honest counterpoint: in higher-end pockets of Eagle, Star, and the Boise Foothills where every comp has a 2- or 3-car garage, removing all covered parking can hurt resale. That is exactly why we frequently recommend converting only one bay of a 2-car garage, or pairing the conversion with a new carport so the property still shows covered parking when it goes back on the market.
Can I convert just half of a two-car garage?
Yes, and in Boise it is one of the most popular configurations we build. You keep one bay for a vehicle, workshop, or storage and convert the other bay into a 200 to 300 square foot studio or efficiency unit. The new interior demising wall between the parked-car bay and the living space must be built as a code-compliant garage-to-dwelling separation, which generally means 5/8-inch Type X gypsum on the garage side, a self-closing 20-minute rated door if there is any pass-through, and continuity of the air and thermal barrier. We also recommend extra acoustic treatment so the tenant does not hear the garage door opener at 6 a.m.
What is the minimum ceiling height for a garage conversion ADU in Boise?
Under the International Residential Code as adopted in Idaho, habitable rooms need a 7-foot minimum finished ceiling height, and bathrooms, hallways, and laundry can be as low as 6 feet 8 inches. Most attached Boise garages from the 1980s onward have 8-foot ceilings, which leaves you 12 inches to absorb ceiling insulation, drywall, lighting cans, and any new floor assembly. Pre-1970s detached garages and a lot of carport-style conversions in the North End and Central Bench can be tighter than that. We laser-measure every plane during the feasibility visit and tell you up front whether the existing height works or whether the project needs a ceiling redesign, a slab depression, or a different ADU strategy entirely.
Do I need new egress windows in a Boise garage conversion?
Almost always, yes. Every room used for sleeping needs an emergency escape and rescue opening sized for at least 5.7 square feet of net clear opening, at least 20 inches wide, at least 24 inches tall, with a sill no higher than 44 inches above the finished floor. Most garages have either no windows or small high-mounted windows that do not meet those numbers. Cutting a new egress opening in a stud-framed garage wall is straightforward; cutting one in a concrete or CMU garage wall (common on older detached garages near Boise State and Garden City) requires a structural lintel and a saw cut that adds cost. We always confirm egress strategy in the design phase, not in the field.
What happens to the garage door opening?
The overhead door, springs, tracks, and operator come out. The opening then gets framed in to match the surrounding wall depth, sheathed, weather-resistive barriered, insulated, and re-clad in siding that matches the rest of the house. The streetscape goal is that someone driving by cannot tell where the door used to be. Two interior layouts are common: a solid infill wall with a couple of standard windows and a new pedestrian entry door, which is the cheapest and gives you the most usable interior wall; or a glazing-forward layout with a wall of windows or a sliding patio door, which brings in light but costs more and limits furniture placement. We will show you elevation renderings of both before you commit.
How much does a garage conversion cost in the Treasure Valley?
Realistic 2026 ranges we are seeing in Ada and Canyon counties: a single-bay studio conversion lands in the low six figures, a full two-car one-bedroom conversion is mid-six-figures, and an oversized 3-car or shop-style detached garage conversion can run higher when full kitchen, bath, and HVAC are added. Per-square-foot, garage conversions in Boise typically come in well below detached new-build ADUs because you are not paying for foundation, framing, or roof. The biggest swing factors are plumbing distance to the main sewer, electrical service capacity, insulation strategy for the existing walls, and finish level. We give you an itemized estimate with allowances rather than a single mystery number.
Is a garage conversion really cheaper than building a detached ADU?
Usually yes, but not always, and that surprises homeowners. A clean, well-built modern attached 2-car garage with healthy slab, healthy walls, and short utility runs converts at roughly 50 to 70 percent of the cost of an equivalent detached new-build ADU. But an older detached garage with a cracked or sloped slab, undersized framing, no frost-depth footings, a tired roof, and a 60-foot trench to the nearest sewer line can end up costing nearly as much as building new, and you still inherit the old building's quirks. We do a real structural and utility assessment first. If conversion is not the right financial move on your specific property, we will tell you and point you to our detached ADU page instead.
What about the slab, frost depth, and foundation under a Boise garage?
This is the single most important hidden-cost area. Boise's frost depth is generally taken as 24 inches, and habitable building footings need to bear below that line. Garages built as habitable-adjacent structures usually meet it; older detached garages, carports converted into garages, and DIY additions often do not. A 4-inch slab with no perimeter footing, no rebar, no vapor barrier, and no edge insulation is fine for parking a truck but is not a code-legal floor for a dwelling unit. Depending on what we find, the remedy ranges from a simple vapor barrier plus rigid-foam plus engineered subfloor over the existing slab, to a structural topping slab, to underpinning sections of the perimeter. We will not assume the slab is good; we open inspection points and verify.
Will my electrical service handle an ADU?
Sometimes, often not. A fully built-out ADU with a heat pump, electric range or induction cooktop, water heater, dryer, and general lighting and outlets typically needs a 60 to 100 amp dedicated subpanel. Many older Boise homes still run on 100-amp main service, and a fair number of homes in the North End, East End, and older Meridian neighborhoods are already close to capacity before you add a dwelling unit. The fix is usually a 200-amp main service upgrade plus a new dedicated ADU subpanel. We do the load calculation during design so the service upgrade, if it is needed, is in the budget from day one rather than a surprise change order.
How do I get plumbing into a detached garage?
You trench. Water supply, sanitary drain, and (depending on the design) a gas line have to get from the main house or the street stub to the converted structure, usually 20 to 60 feet across a yard, often under a driveway or hardscape. In Boise, the trench has to be deep enough to keep water lines below frost (typically 30 inches or more for safety) and the drain has to maintain a quarter-inch-per-foot fall to the existing sewer lateral. If the garage slab sits at or below the elevation of the sewer main, gravity drainage will not work and a sealed ejector pump system is required for the bathroom and kitchen waste. We probe and shoot grades before pricing, because guessing here is how garage conversions blow their budget.
How does Boise's energy code affect the conversion?
Boise sits in IECC Climate Zone 5, which means the converted space is treated as new conditioned space and has to meet meaningful insulation, window U-value, and air-tightness targets. In practice, that usually means R-21 in 2x6 walls or closed-cell spray foam on existing 2x4 walls, R-49 at the ceiling/roof line, R-10 continuous under the new floor or at the slab edge, U-0.30 or better windows, and a blower-door test result that meets the current air-leakage threshold. Closed-cell spray foam is popular in garage conversions because it solves insulation, air sealing, and vapor control in one application without losing interior dimension to furred-out studs. Verify current code with your jurisdiction at design time, because Idaho updates the adopted code periodically.
When is a garage conversion the wrong choice?
Be honest with yourself if any of the following apply: the garage has settled, sloped, or significantly cracked the slab; the existing walls are uninsulated single-wall or board-on-batten construction that effectively has to be rebuilt; the roof is at the end of its life; the structure is too close to the property line to satisfy current setback rules for habitable space; the lot already has a detached ADU or other dwelling unit that caps your zoning; you genuinely need that covered parking for resale in a high-end neighborhood; or your real goal is two bedrooms plus a real living room, which is hard to fit inside a single-car footprint. In those cases we recommend looking at a detached new-build ADU, a basement conversion, or an above-garage second-story unit instead.
A garage conversion is not the only path to an accessory dwelling unit. Depending on your lot, budget, goals, and what we find on the feasibility visit, another type may be the better fit. Visit our ADU construction hub for a complete overview, or jump straight to the alternatives below.
Detached ADU (Backyard Cottage)
Standalone structure — maximum privacy and design flexibility
Basement ADU Conversion
Convert existing basement space into a cost-effective living unit
Above-Garage ADU
Second-story addition on an existing or new garage
Attached ADU Addition
Home addition with separate entrance and independent systems
Detached vs. Garage Conversion
Side-by-side cost, schedule, and resale comparison
ADU Construction Overview
Every Iron Crest ADU type in one place
The fastest way to find out whether your specific garage is a good conversion candidate is to get a free in-home estimate from Iron Crest Remodel. We will measure the existing structure, open inspection points on the slab and walls, photograph the electrical service, shoot grades to the main sewer cleanout, and give you a written, itemized scope and budget rather than a guess.
Get A Free In-Home Estimate
Written, itemized scope and budget — no obligation
Contact Iron Crest Remodel
Call (208) 779-5551 or send us a message
ADU Construction Hub
Every Iron Crest ADU type and how they compare
Guides & Planning Articles
Iron Crest planning resources for Treasure Valley homeowners
Service Areas / Regions
Cities and neighborhoods we cover across the Treasure Valley
Detached vs. Garage Conversion
Decide between the two most common ADU paths
Iron Crest Remodeling Group LLC (Iron Crest Remodel) is a licensed and insured Idaho residential contractor, RCE-6681702, serving Boise and the broader Treasure Valley. Office hours are Monday through Friday, 7 a.m. to 6 p.m. Every project includes our 5-year workmanship warranty and a free in-home estimate. Call (208) 779-5551 to schedule.
The following government agencies, industry organizations, and official resources provide additional information relevant to your remodeling project.
Ready For A Garage Conversion?
Iron Crest Remodel handles every step — feasibility, design, permits, construction, and final inspection. Get a free, no-obligation in-home estimate for your Boise garage conversion ADU.