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 living space — complete with a kitchen or kitchenette, bathroom, sleeping area, and a dedicated entrance. Rather than building a new structure from the ground up, you repurpose a building that already has a foundation, walls, and roof, which dramatically reduces both cost and construction time compared to every other ADU type.
Garage conversions are the most affordable ADU option available to Boise homeowners because the structural shell already exists. The foundation slab, framed walls, and roof assembly eliminate the most expensive phases of new construction, allowing your budget to focus on what matters: insulation, plumbing rough-in, electrical upgrades, HVAC, and interior finishes that create a comfortable, code-compliant home. A typical converted garage yields 300 to 600 square feet of finished living space, depending on whether you're working with a single-car, two-car, or oversized three-car garage.
In Boise's growing ADU market, garage conversions are especially popular among homeowners who want to generate rental income quickly, create an in-law suite for aging parents, or add a private guest house without the 6-to-12-month timeline of ground-up construction. The conversion process typically takes just 2 to 4 months from permit approval to certificate of occupancy, making it the fastest path to a functional accessory dwelling unit on your property.
Iron Crest Remodel has completed garage-to-ADU conversions across the Boise metro area, from compact single-bay studios in the North End to full two-car conversions in Meridian and Eagle. Every project begins with a thorough structural assessment of your existing garage to determine feasibility, identify code compliance requirements, and develop an accurate scope and budget before any work begins.
The economics of a garage conversion are straightforward: you already own the most expensive components of a building. The foundation, framing, exterior walls, and roof represent 40% to 60% of new construction costs. By converting an existing garage, you bypass $40,000 to $80,000 in structural work that every other ADU type requires. Here's exactly where those savings come from.
Existing Structure Reduces Cost by 40–60%
Your garage already has a poured slab, standing walls, and a weathertight roof. These three elements represent the largest single cost in any new construction project. A detached ADU of similar size would require $50,000 to $80,000 just for the foundation, framing, and roofing phases. With a garage conversion, your investment goes directly into making the interior livable — insulation, mechanical systems, and finishes.
Faster Timeline Means Lower Labor Costs
Because the shell already exists, construction focuses entirely on interior buildout. A typical Boise garage conversion moves from permit approval to certificate of occupancy in 2 to 4 months — roughly half the time of a detached ADU. Faster completion means lower labor costs, less disruption to your property, and earlier rental income or occupancy.
No New Foundation Needed
Your garage slab is already poured and cured. While it may need a moisture barrier, insulation, or a topping slab in some cases, you avoid the full cost of excavation, formwork, concrete pouring, and curing time. In Boise's clay-heavy soils, foundation work alone can run $15,000 to $30,000 for a new structure — a cost that garage conversions eliminate almost entirely.
Utilities Already Nearby
Most Boise garages already have electrical service with a subpanel or dedicated circuits, and attached garages share a wall with the main home's plumbing and HVAC systems. Even detached garages are typically within 20 to 40 feet of existing utility connections. This proximity reduces utility rough-in costs by $3,000 to $8,000 compared to running new service lines to a structure built at the rear of the lot.
No New Roof Required
The existing roof assembly — trusses, sheathing, and roofing material — stays in place. As long as the roof is in sound condition, you simply add insulation to the ceiling or attic space above. A new roof for a detached ADU adds $8,000 to $20,000 to the project cost, a line item that garage conversions avoid completely.
Garage conversion costs in Boise depend on the garage size, condition of the existing structure, scope of plumbing and electrical work, and your finish level. Below are typical price ranges based on projects completed in the Treasure Valley market during 2025 and 2026.
| Garage Type | Sq Ft Range | Typical Cost |
|---|---|---|
| 1-Car Garage | 250 – 350 sq ft | $80,000 – $120,000 |
| 2-Car Garage | 400 – 600 sq ft | $100,000 – $150,000 |
| 3-Car Garage | 550 – 750 sq ft | $120,000 – $180,000 |
Per square foot cost: $250 – $350 depending on scope and finishes.
Key Cost Items
Insulation & Weatherization
$4,000 – $8,000
HVAC (Mini-Split or Ducted)
$4,000 – $8,000
Plumbing Rough-In (Bath + Kitchen)
$8,000 – $18,000
Electrical Panel & Wiring
$5,000 – $12,000
Subfloor / Slab Prep
$2,000 – $6,000
Windows (Egress + Natural Light)
$3,000 – $8,000
Entry Door & Garage Door Infill
$3,000 – $8,000
Permits & Fees
$2,000 – $6,000
A garage conversion is not the right choice for every property. Here's an honest assessment of the advantages and limitations based on Boise's building codes, climate, and real estate market.
Advantages
- Lowest ADU cost per square foot of any conversion type
- Fastest build timeline — 2 to 4 months of active construction
- Existing structure eliminates foundation, framing, and roofing phases
- No new foundation required — slab already poured and cured
- Less permitting complexity — no new setback or lot coverage calculations
- Often can retain exterior appearance — neighbors rarely notice the change
Limitations
- Loss of garage parking and enclosed storage space
- May need parking alternatives — driveway expansion, carport, or street parking
- Limited by existing garage footprint — cannot easily expand square footage
- Garage door opening requires wall infill — structural and aesthetic consideration
- Often lower ceiling height — 8′ typical, tight after insulation and finish layers
- May need structural reinforcement for residential loads on thinner garage slabs
Converting a garage into a living space involves more than finishing the interior. Several structural and mechanical systems require careful planning to meet Boise's building codes and create a durable, comfortable home.
Foundation & Slab Upgrades
Standard garage slabs are 4 inches thick with minimal reinforcement, while residential living spaces require a more robust foundation. Iron Crest Remodel evaluates every garage slab for thickness, cracking, moisture intrusion, and load capacity before beginning work. A vapor barrier or moisture-resistant membrane is installed over the existing slab to prevent moisture migration into the living space. Rigid foam insulation (R-10 minimum per code) is added above the slab before the finished subfloor is installed. If the slab is inadequate, options include a structural topping slab of 1.5 to 2 inches of fiber-reinforced concrete, localized reinforcement at bearing points, or a full overlay. Most Boise garages built after 1985 have slabs that meet minimum requirements with only minor upgrades.
Garage Door Infill
The garage door opening is the largest single modification in any conversion. The existing door and track system are removed, and the opening is framed with standard 2x6 lumber to match the surrounding wall depth. New siding is applied to the exterior to match the existing garage facade — when done correctly, the former garage door location is nearly invisible from the street. The two primary interior approaches are framing a full wall with standard windows and a new entry door, or installing a wall of windows and/or a sliding glass door to maximize natural light. A full wall infill costs $3,000 to $5,000, while a window-wall approach costs $5,000 to $8,000. Both approaches require a structural header to carry the load previously supported by the garage door track and lintel.
Ceiling Height Requirements
The International Residential Code requires a minimum 7-foot ceiling height for habitable rooms. Most standard garages provide 8-foot ceilings, but the addition of ceiling insulation, drywall, and a subfloor assembly can reduce finished height by 4 to 8 inches. Garages with ceiling heights below 8 feet 6 inches require careful assembly planning to maintain the 7-foot minimum after all finish layers are applied. Iron Crest Remodel measures and documents existing ceiling heights during the feasibility assessment to ensure every room meets code before construction begins.
HVAC Integration
Ductless mini-split heat pumps are the most common and cost-effective HVAC solution for Boise garage conversions. A single-zone mini-split provides both heating and cooling for $4,000 to $7,000 installed, operates independently from the main home's system, and achieves excellent energy efficiency. In Boise's climate, a cold-climate rated mini-split (effective to -15°F) handles winter heating without supplemental electric resistance heat. For attached garages where ducting from the main house is accessible, extending the existing HVAC system may be an option, though this requires careful load calculation to ensure the main system can handle the additional square footage.
Plumbing Access
Adding a bathroom and kitchenette to a garage requires new water supply and drain lines. The plumbing approach depends on the garage's proximity to existing sewer and water connections. Attached garages that share a wall with the kitchen or bathroom often allow direct tie-ins at reasonable cost. Detached garages may require trenching across the yard to connect to the main sewer line, adding $3,000 to $8,000 to the plumbing budget. Drain slope is critical — garage slabs are typically at or below the main home's floor level, so an ejector pump or up-flush system may be needed if gravity drainage is not feasible.
Electrical Capacity
A full ADU with kitchen appliances, HVAC, lighting, and outlets requires 60 to 100 amps of dedicated electrical service. Most existing garages have only 15 to 30 amps of capacity on a shared circuit with the main home. Upgrading to a dedicated subpanel in the ADU is standard practice. If the main home's service panel is at capacity (common in older Boise homes with 100-amp panels), a main panel upgrade to 200 amps may be necessary before the ADU subpanel can be added — budget $2,000 to $4,000 for this upgrade.
Garage conversions must meet the same building codes as any new residential living space. Iron Crest Remodel ensures full compliance with all City of Boise and Idaho state requirements throughout the conversion process.
IRC Habitable Space Standards
All habitable rooms must meet International Residential Code standards for minimum room dimensions, natural light (8% of floor area in glazing), and natural ventilation (4% of floor area or mechanical equivalent). Habitable rooms require a minimum of 70 square feet of floor area with no dimension less than 7 feet. Kitchens are exempt from the minimum area requirement but must meet all other habitability standards. Every room must have adequate artificial lighting in addition to natural light requirements.
Egress Windows
Every sleeping room must have at least one emergency escape and rescue opening (egress window) with a minimum net clear opening of 5.7 square feet, a minimum width of 20 inches, and a maximum sill height of 44 inches above the floor. Since garages typically have small or no windows, adding code-compliant egress windows is a standard part of every conversion. This often involves cutting new openings in the garage walls and installing properly sized windows with wells if below grade.
Ceiling Height
Habitable rooms require a minimum 7-foot ceiling height over at least 50% of the required floor area. Bathrooms, hallways, and laundry rooms require 6 feet 8 inches minimum. Most standard garages provide 8-foot ceilings, which leaves adequate clearance after insulation, drywall, and flooring assemblies are installed. Garages with ceiling heights below 8 feet 6 inches require careful assembly planning to maintain code compliance after all finish layers are applied.
Thermal Envelope Requirements (Climate Zone 5)
Boise is in IECC Climate Zone 5, which requires significant insulation for energy code compliance. Converted garage walls need R-21 cavity insulation (typically 2x6 framing with fiberglass batts or spray foam), and ceilings require R-49 insulation. Slab edge insulation of R-10 is required. Windows must be rated U-0.30 or lower. An air barrier and blower door test (maximum 3 ACH50) are also required. These standards ensure the ADU is energy-efficient and comfortable through Boise's cold winters and hot summers. Most existing garage walls are uninsulated 2x4 framing, so converting to R-21 may require furring out walls or using closed-cell spray foam, which achieves higher R-value per inch.
Fire Separation (Attached Garages)
If your garage is attached to the main home, the shared wall must maintain its fire separation rating even after the garage is converted to living space. The IRC requires a minimum 1-hour fire-resistance rating for the wall separating an attached garage from living space, typically achieved with 5/8-inch Type X drywall on both sides. Once the garage is converted to habitable space, the fire separation requirement may change depending on the occupancy classification — Iron Crest Remodel works with the City of Boise plan reviewers to determine the correct fire separation requirements for your specific configuration. Smoke and carbon monoxide detectors must be hardwired with battery backup and interconnected throughout the converted unit.
Not every garage is an ideal conversion candidate. The following property types consistently produce the best results in terms of cost efficiency, livability, and return on investment in the Boise market.
2-Car Garages With One Bay Converted
The most popular conversion approach in Boise. Convert one bay into a 200–300 sq ft studio ADU while retaining the other bay for parking or storage. This preserves some covered parking, reduces construction costs, and still creates a functional rental unit generating $700 to $1,000 per month. The dividing wall between bays must meet fire separation and sound transmission requirements.
Detached Garages Near Alley Access
Detached garages that face an alley or rear lane are excellent conversion candidates because they offer a separate entrance path for the ADU tenant without crossing through the homeowner's yard. Alley access also simplifies utility connections and provides natural separation between the main home and the ADU, increasing privacy for both parties. Many older Boise neighborhoods — the North End, Hyde Park, and Central Bench — have this configuration.
Oversized Garages in Older Neighborhoods
Homes built in the 1950s through 1970s in Boise's established neighborhoods often have oversized garages — 24 by 28 feet or larger — that were built for workshop space, boat storage, or multiple vehicles. These larger footprints yield 500–700 sq ft ADUs with comfortable one-bedroom layouts, full kitchens, and generous living areas. The additional square footage makes these conversions especially attractive for long-term rental income.
Homes With Street Parking or Carport Alternatives
If your property has ample street parking, a long driveway, or space for a carport, the loss of enclosed garage parking is minimal. The City of Boise does not require additional off-street parking for ADUs, so homeowners with alternative parking options can convert the full garage without regulatory complications. A simple carport ($3,000–$8,000) provides covered parking at a fraction of the cost of the garage space you're repurposing into a high-value living unit.
Do I need a permit to convert my garage into an ADU in Boise?
Yes. A garage-to-ADU conversion in Boise requires a building permit from the City of Boise Planning and Development Services department. You will also need separate trade permits for plumbing, electrical, and mechanical work. The City reviews plans for compliance with zoning requirements, building codes, energy codes, and fire safety standards before issuing a permit. Iron Crest Remodel handles the entire permit application and inspection process on your behalf.
How long does a garage conversion take in the Boise area?
A typical Boise garage conversion takes 2 to 4 months of construction time after permits are approved. The permit review process adds 4 to 8 weeks. Single-car garage conversions on the simpler end can finish in as little as 8 weeks of active construction, while larger two-car conversions with premium finishes may take 12 to 16 weeks. Overall, plan for 4 to 6 months from permit application to move-in.
Will I lose too much property value by eliminating my garage?
In Boise’s current market, the rental income potential and added livable square footage from a garage conversion ADU typically outweigh the loss of covered parking. A well-executed garage conversion adds $60,000 to $120,000 in assessed property value while generating $800 to $1,200 per month in rental income. Driveway parking, carports, or a new detached parking structure can offset the loss of enclosed garage space at a fraction of ADU construction cost.
Can I convert just half of a two-car garage?
Yes. Partial garage conversions are permitted in Boise and can be an excellent compromise. You can convert one bay of a two-car garage into a studio or one-bedroom ADU while retaining the other bay for parking or storage. This approach reduces construction costs, preserves some covered parking, and still creates a functional 200 to 300 square foot living space. The dividing wall must meet fire separation and sound transmission requirements per code.
What is the minimum ceiling height for a garage conversion ADU in Boise?
Boise follows the International Residential Code, which requires a minimum ceiling height of 7 feet for habitable rooms and 6 feet 8 inches for bathrooms, hallways, and laundry areas. Most standard garages have 8-foot ceilings, which provides adequate clearance after insulation and finish materials are added. If your garage has a ceiling height below 8 feet, the insulation and drywall assembly may reduce the finished height close to the minimum, requiring careful planning of mechanical systems and finish layers to maintain compliance.
A garage conversion isn't the only option. Depending on your lot, budget, and goals, another ADU type may be a better fit. Explore the alternatives below or visit our ADU Construction hub for a complete overview.
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
Plan your garage conversion with the help of these related guides and services from Iron Crest Remodel.
ADU Construction Cost Guide
Complete ADU pricing for all types in Boise
ADU Permits & Regulations
Zoning, setbacks, and the full permit process
ADU Construction Timeline
Phase-by-phase schedule from design to move-in
ADU Design Ideas
Layout inspiration and finish options for Boise ADUs
ADU Materials Guide
Best materials for durability and value in Idaho’s climate
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The following government agencies, industry organizations, and official resources provide additional information relevant to your remodeling project.
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