Detached ADU vs Garage Conversion in Boise
A side-by-side comparison of the two most popular ADU options for Boise and Treasure Valley homeowners — covering real costs, timelines, Ada County zoning reality, utilities, parking trade-offs, rental income logic, and which option wins for your lot and goals.
Boise homeowners exploring an accessory dwelling unit face one fundamental fork early in planning: build a detached ADU from the ground up in the backyard, or convert an existing garage into a finished, code-compliant living unit. Both create a legal second dwelling on your property, and both can be excellent investments in the Treasure Valley's tight housing market. But they diverge sharply on cost predictability, timeline, what your lot and zoning will allow, parking, privacy, and long-term return. Choosing the wrong path for your specific parcel can cost months of schedule and tens of thousands of dollars in change orders.
This page exists to compare those two options against each other — honestly, with the trade-offs most contractors leave out. It is not a sales pitch for one path. The right answer genuinely depends on your structure, your lot, your budget ceiling, and what you want the unit to do for you over the next ten years. We have designed and built both types across central Boise, the North End, the Bench, and surrounding Ada County, and the pattern is consistent: the “cheaper” option is not always cheaper once the existing garage is actually inspected, and the “expensive” option is often the more predictable one.
One framing point before the details. Boise ADU demand is driven by a constrained rental market, infill-friendly neighborhoods like the North End and Bench, foothills lots with room to build, and a steady stream of homeowners housing family or generating income. That demand is real — but the regulations that govern what you can build are set by the City of Boise or Ada County and are updated periodically. Every zoning, setback, parking, owner-occupancy, and size figure discussed here should be treated as something to verify against current code for your exact parcel before you design or budget.
For a deeper look at either path on its own, see our detached ADU guide and our garage conversion guide. This page stays focused on the head-to-head decision.
Before comparing numbers it helps to define both options precisely, because the words are used loosely and the distinction drives every downstream cost.
Detached ADU
A standalone dwelling built new on the same lot as the primary residence, structurally independent of the main house with its own foundation, walls, roof, entrance, and typically its own utility connections. It is a complete small home: kitchen, bathroom, sleeping and living space, and its own heating and cooling. Because nothing is being reused, you control every assembly — foundation type, insulation values, ceiling height, layout, window placement, and aging-in-place features. That control is the central advantage of the detached path and the reason its budget is more predictable than a conversion once a project is underway.
Garage Conversion ADU
Converting the volume of an existing garage — attached or detached — into a finished, permitted dwelling unit by adding insulation, interior walls, egress, plumbing, electrical, mechanical, finished floor over the slab, and the infill where the garage door used to be. The defining trait is reuse: you are inheriting a foundation, slab, walls, and roof that were built for vehicle storage, not human habitation. When those existing elements meet the standards required for a heated, occupied dwelling, the conversion is genuinely cheap. When they do not — uninsulated slab, no frost footing, low ceiling, no code-compliant egress, undersized electrical service — the upgrades to bring them up to standard are exactly where projected savings evaporate.
The single most important idea on this page: a garage conversion is a bet that the existing structure is reusable. A detached ADU removes that bet by building everything new. Most of the rest of this comparison is really about how to evaluate that bet for your specific garage.
Cost is the most decisive factor for most Boise homeowners, and it is also the most misunderstood. The headline is true: a garage conversion is usually cheaper than a detached ADU when the existing structure and slab and utilities are genuinely reusable. The trap is that the qualifier is doing enormous work, and homeowners routinely budget for the headline and not the qualifier.
Detached ADU — Cost Profile
A ground-up project: new foundation (slab or crawlspace with frost footings), full framing, roofing, exterior finishes, trenched utility runs, insulation to current energy code, drywall, flooring, a full kitchen and bath, a dedicated HVAC system, and its own electrical sub-panel or service. Higher total cost than a conversion in nearly every case. The offsetting advantage is predictability — because nothing is being discovered behind a finished surface or under an old slab, the scope is known at design time and surprises during construction are far less common.
Garage Conversion — Cost Profile
Reuses the foundation, walls, and roof, which removes a large block of structural and shell cost. Budget covers insulation, interior partitions, garage-door infill, egress modifications, plumbing for a bath and kitchen, electrical upgrades, HVAC, a finished floor system over the concrete, and interior finishes. Cheaper than detached only to the extent the existing structure does not need upgrading. The cost risk is the inverse of detached: lower base cost, higher variance, because the real scope is partly hidden until walls are opened and the slab and footings are evaluated.
We do not publish fixed prices on this page because Treasure Valley material and labor costs move and every parcel is different — a fabricated number would mislead you. Instead, here are the specific line items that most often turn a “cheap” garage conversion into something approaching detached-ADU cost. Walk this list against your own garage before you decide:
Slab and footing: garage slabs are often thin, uninsulated, and lack the frost-protected footing a heated dwelling needs in Idaho. Underpinning, insulating, or partially replacing the slab is a major hidden cost.
Ceiling height: many older Boise garages have low headroom. Once you add floor buildout over the slab plus ceiling insulation and finish, usable height can fall below what feels livable — sometimes requiring roof or floor changes.
Egress: a sleeping room needs a code-compliant egress opening. Cutting an egress window or door into a concrete or masonry garage wall is significantly more expensive than framing it into a new detached wall.
Insulation and energy code: garage walls and roof were rarely built to the insulation standard required for a conditioned dwelling, so walls and roof assemblies often need to be furred, re-sheathed, or rebuilt.
Electrical service: a second dwelling adds load. Older Boise homes frequently have service that must be upgraded before a conversion or a sub-panel can be added.
Garage-door infill and exterior: removing the door, framing and insulating the wall, matching siding, and adding code windows is a real assembly, not a patch.
Moisture and floor: sealing the slab and building an insulated, level finished floor over concrete is standard for conversions and routinely underestimated.
The pattern we see repeatedly in Boise: a homeowner expects a conversion to come in at roughly half the cost of detached, then the slab, ceiling height, egress, and electrical findings stack up until the conversion lands much closer to detached — but with a smaller, lower-ceiling unit and lost parking at the end of it. That is the single most important cost insight on this page. If three or more of the items above apply to your garage, get both options scoped before assuming the conversion is the value play. For broader cost context across all ADU types, see the ADU construction overview.
Zoning often decides the detached-versus-conversion question before cost does, and it is the area where homeowners get the most outdated information from blogs, neighbors, and even older permit experiences. ADU rules in the Treasure Valley are set by the jurisdiction your parcel sits in — the City of Boise for most in-city lots, or Ada County Development Services for unincorporated areas — and those rules are updated periodically. Nothing in this section should be treated as a fixed number for your property. Treat each item below as a question to verify against current code for your exact parcel and zoning district.
Minimum lot size
Many jurisdictions tie ADU eligibility to a minimum lot size or to the underlying zoning district. A garage conversion that reuses an existing footprint can sometimes proceed on a lot that would not support a new detached structure under current standards. Verify the minimum that applies to your parcel today.
Setbacks
New detached structures must meet side and rear setbacks. An existing garage already sitting near a property line may be allowed to be converted in place even where a new building could not be located there — a real advantage for conversions on tight lots. Confirm current setbacks and any legal non-conforming status.
Maximum ADU size
ADU size is commonly capped, sometimes as an absolute square footage and sometimes as a ratio of the main house. This can limit a detached unit’s rental and resale ceiling and is one reason building to the cap rather than the budget often makes sense. Verify the current maximum for your zone.
Total lot coverage
House plus garage plus any new structure usually must stay under a coverage limit. A conversion adds no new footprint, so it does not consume coverage; a detached unit does. On a near-maxed lot this single rule can rule out detached entirely. Confirm current coverage limits.
Owner-occupancy
Some jurisdictions require the owner to live in the primary residence or the ADU. This affects your ability to rent both units and is the kind of rule that has changed in many cities in recent years — verify whether an owner-occupancy requirement currently applies.
Parking
Whether you must provide off-street parking for an ADU — and whether converting a garage triggers a replacement-parking requirement — depends on current code and zone. This rule directly interacts with the garage-conversion decision and changes frequently. Verify it before designing.
One ADU per lot
Most lots are limited to a single ADU. If you already have a unit — or an existing structure that could be deemed one — a second may not be permitted. Confirm how many units your parcel can legally support under current code.
The practical takeaway: on constrained lots, the zoning advantage frequently favors the garage conversion, because reusing an existing footprint can sidestep setback and coverage limits that would block a new detached structure. On larger Boise and foothills lots with room to build, zoning is rarely the binding constraint and the decision returns to cost, privacy, and return. Either way, the only reliable source for what your parcel allows is the current City of Boise or Ada County code — we confirm the applicable rules with the jurisdiction during design rather than relying on prior projects, because the code does move.
Utilities are where the cost gap between the two options is most consistent and most often underestimated. The structure and finishes get the attention; the water, sewer, and power are what quietly move the budget.
Electrical
A detached ADU needs power run to a new structure and its own sub-panel or dedicated panel sized for a full dwelling. A garage conversion ties into existing wiring with shorter runs, but a second dwelling adds load that the existing service may not carry — many older Boise homes need a service upgrade before either a conversion or a sub-panel is safe and code-compliant. That upgrade is the most common “surprise” on conversion projects and partially closes the cost gap with detached.
Water & Sewer
A detached ADU usually requires trenched water and sewer lines from the main house or street, and depending on the utility provider and jurisdiction may require a separate tap or connection — the longer the run and the more separate the connection, the higher the cost. A garage conversion typically extends existing house plumbing a short distance, which is materially cheaper. Tap and connection requirements and fees are set by the utility and jurisdiction and change, so confirm current requirements during design rather than assuming the rules from a past project still apply.
Separate vs Shared Metering
Separate metering — the ADU on its own electric, gas, or water meter — is the quiet decision that affects rentability and appraisal. Separately metered units are easier to rent (the tenant pays their own utilities), simpler to manage, and often valued more favorably by appraisers as a true income unit. Detached ADUs lend themselves to separate metering more naturally; garage conversions are more often left on shared house utilities to save cost, which simplifies construction but complicates billing and can soften how the unit appraises. Metering rules and fees are utility- and jurisdiction-specific and change — verify current options before committing.
Your chosen path sets both the available square footage and how quickly you can move a tenant or family member in. These differences cascade into layout, bedroom count, ceiling feel, and livability.
Size & Layout
A detached ADU is designed to a layout, within the size cap your zoning allows — studio, one-bedroom, or two-bedroom, with a real living area and a layout that can be optimized for rent or for aging in place. A garage conversion is constrained by the existing footprint of the garage, which usually caps it at a studio or one-bedroom. Ceiling height is the constraint homeowners feel most: a typical garage has limited headroom, and once a finished floor goes over the slab and insulation plus finish go below the roof, the resulting room can feel tight in a way a purpose-built detached unit never does. If a two-bedroom or generous-ceiling unit is the goal, detached is usually the only path that delivers it.
Construction Timeline
A detached ADU is a longer build because it includes foundation, framing, roofing, and exterior before interior work even starts. A garage conversion is faster because it skips most of the shell and moves more directly into insulation, interior partitions, rough-in trades, drywall, and finishes — provided the structural surprises above do not surface. The honest caveat: a conversion that uncovers slab, footing, or egress problems can lose its timeline advantage in change orders and re-inspections. Use any duration as a planning estimate, not a guarantee, and confirm current City of Boise or Ada County review timelines when you submit.
Beyond cost and schedule, permit complexity and privacy each shape your project experience and your long-term satisfaction with the unit.
Permit Complexity in the Treasure Valley
A detached ADU triggers full new-construction review: site plan with setbacks and coverage, structural details, and separate trade permits. A garage conversion is generally a change-of-use plus trade permits, and the already-approved shell tends to streamline review. Both must satisfy current ADU zoning for your parcel, and an unpermitted conversion is one of the costliest mistakes a Boise homeowner can make — it undermines value and complicates resale and financing. Verify current permit requirements, fees, and review timelines with the City of Boise or Ada County when you apply, because fee schedules and procedures are revised over time.
Privacy & Sound Separation
A detached ADU is physically separate — its own walls, roof, and an airspace gap from the main house — which delivers the sound isolation and visual privacy that tenants and family members value, and which supports longer tenancies and stronger furnished-rental performance. An attached-garage conversion shares walls with the house, so footstep and conversation noise transfer in both directions and privacy is reduced for everyone. A detached-garage conversion sits between the two: more private than attached, but still inheriting a structure built lighter than a purpose-built dwelling. For long-term rental use, this separation difference is one of the strongest arguments for the detached path.
Both options can produce rental income and add resale value in Boise's tight market, but the return math behaves differently and we deliberately avoid fabricated dollar figures here — rents and appraised values are market-, season-, and parcel-specific, and a made-up number would only mislead your planning. Instead, here is the reasoning to apply with current local comparables from a property manager or appraiser.
Detached ADU — Return Logic
Higher rent ceiling: larger, more private, true separate-dwelling appeal supports premium long-term and furnished rents
Stronger appraisal treatment when separately metered and clearly an independent income unit
Lower vacancy risk because it lives like a home, not a converted space — longer tenancies
Higher upfront cost, but more predictable budget and a higher long-run income and resale ceiling
Best when the goal is maximizing income and resale over a 10-year horizon
Garage Conversion — Return Logic
Lower entry cost can mean a faster simple payback — if the structure was genuinely reusable
Lower rent ceiling due to smaller footprint, lower ceilings, and less privacy
Some appraisers discount for lost garage; resale impact depends on the local buyer pool
Return advantage erodes fast if slab, egress, ceiling, or electrical upgrades stack up
Often strongest for family use and proximity rather than premium income
The honest ROI summary: a garage conversion can have a faster simple payback because you invest less — but only if the conversion stayed cheap. A detached ADU costs more and pays back over a longer horizon, but it carries a higher income and resale ceiling and a more predictable budget. Run both with real local comparable rents and a contractor-scoped cost for your specific structure; averages and online calculators routinely flatter the conversion by ignoring the upgrade line items.
Parking is the most overlooked factor in this decision and has real consequences for daily life, resale, and neighbor relations in Boise.
Detached ADU
Your garage stays intact and all existing covered parking is preserved. The ADU sits separately on the lot without touching the driveway or street parking. On Boise homes where the garage is used daily — and through Idaho winters that means snow-free, frost-free vehicles — keeping the garage is a meaningful quality-of-life and resale advantage that the detached path protects.
Garage Conversion
Converting eliminates that covered parking permanently; vehicles move to the driveway or street. Whether code requires you to replace that parking depends on current City of Boise or Ada County rules for your zone — verify before designing. Beyond code, many local buyers expect a garage, so the conversion can soften resale appeal. A carport or expanded driveway can partially offset the loss at added cost but rarely replaces an enclosed garage in an Idaho winter.
The resale nuance worth stating plainly: in much of the Treasure Valley, removing the only garage to gain a small unit is a trade some buyers will not make, which can narrow your future buyer pool even if the unit itself adds value. In dense, walkable pockets of the North End and downtown-adjacent Boise, garage parking matters less to the typical buyer, so the conversion trade-off is softer there. Weigh the parking loss against your neighborhood, not a generic rule.
Where your home sits in the Treasure Valley often decides this question independently of budget. A garage conversion adds essentially no new footprint, so it can succeed on lots a detached unit cannot physically or legally fit. A detached ADU needs both buildable area under current setbacks and coverage and realistic construction access — a way to bring materials, concrete, and equipment to the build location.
North End & Bench alley lots
Many North End and Bench lots have rear alley access. An alley is a strong signal in favor of a detached ADU: it provides a discreet separate entrance and address feel that tenants pay for, and it gives the crew a staging and access path that does not run through the main yard or driveway. This is a large part of why detached-ADU interest concentrates in alley-served Boise neighborhoods. Confirm any alley access easement and current setbacks for your parcel.
Tight interior Bench & older infill lots
On narrow, deep lots with no alley and a constrained side yard, getting equipment and materials to a backyard build is difficult and expensive, and setbacks or coverage may forbid a new structure outright. On these lots a garage conversion is frequently the only feasible ADU, precisely because it adds no footprint and needs no new access path.
Foothills & larger suburban Ada County lots
Foothills and larger suburban lots usually have the room and access for a detached unit, so the decision returns to cost, privacy, and return rather than feasibility. Slope, grading, and longer utility runs become the cost variables to scope on these properties. Explore service areas on our regions page.
This table summarizes the qualitative trade-offs so you can compare both options at a glance. Cost, rent, and value are expressed as relationships rather than fabricated dollar figures, because real numbers are parcel- and market-specific and must come from a scoped estimate and current local comparables.
| Factor | Detached ADU | Garage Conversion |
|---|---|---|
| Upfront Cost | Higher | Lower — only if structure is reusable |
| Cost Predictability | More predictable (all new) | Higher variance (hidden scope) |
| Construction Timeline | Longer (full shell) | Shorter, unless surprises surface |
| Size Potential | Up to zoning size cap | Limited to existing footprint |
| Bedroom Options | Studio, 1BR, or 2BR | Usually studio or 1BR |
| Ceiling / Feel | Designed, can be generous | Often tight after buildout |
| Permit Path | Full new construction | Change-of-use + trades |
| Rent Ceiling | Higher | Lower |
| Appraisal Treatment | Often more favorable | May be discounted for lost garage |
| Parking | Garage preserved | Garage parking lost |
| Privacy & Sound | Full separation | Shared/close — less private |
| Utilities | New runs, own sub-panel/meter | Tie into house, possible service upgrade |
| Tight / No-Alley Lots | May be infeasible | Often the only feasible option |
| Best For | Max income & resale, privacy | Budget, speed, proximity, family |
The right answer is situational. Here is how the decision usually breaks down for common Treasure Valley homeowner scenarios — always validated against your scoped costs and current code, not assumed.
Detached ADU Usually Wins When…
The primary goal is maximizing long-term rental income and resale ceiling
You want a private, furnished-rental-grade unit or a true two-bedroom layout
You rely on your garage for parking and refuse to give it up
The lot has buildable area, access, and zoning headroom for a new structure
An alley provides a separate entrance and clean construction access
Budget predictability matters more than the lowest possible entry cost
You want a one-level aging-in-place design with full control of the layout
Garage Conversion Usually Wins When…
The budget ceiling is firm and lower than a detached build requires
The existing garage has a sound slab, footing, roof, and adequate height
You do not depend on the garage for daily vehicle parking
The lot is too tight, landlocked, or constrained for a new structure
Setbacks or coverage limits would block a new detached building
The goal is housing a family member quickly and close to the main house
A faster, lower-risk path matters more than the highest income ceiling
If your situation lands in both columns — common — the deciding factor is almost always the condition of the existing garage and what your current zoning allows. That is exactly what an in-home estimate resolves.
These are the recurring errors we see Boise homeowners make when choosing between the two paths. None of them are about craftsmanship — they are decision mistakes made before the project starts.
Budgeting the garage conversion at the headline “half the cost” before anyone has inspected the slab, footing, ceiling height, egress, and electrical service. This is the single most common and most expensive mistake.
Treating zoning as fixed. Setbacks, coverage, parking, owner-occupancy, and max-size rules change; a design that pencils under last year’s code may not match the code in effect when you submit. Always verify current City of Boise or Ada County code for your parcel.
Building unpermitted to save time or money. An unpermitted unit undermines appraised value, complicates financing, and can derail a future sale — it is not a shortcut, it is a liability.
Ignoring parking loss and its resale impact in neighborhoods where buyers expect a garage and Idaho winters make covered parking valuable.
Sizing a detached ADU to the budget instead of to the zoning cap and the rent/resale ceiling, leaving long-term income permanently on the table.
Forgetting separate metering until after design. Retrofitting metering is harder and costlier than planning it in, and it affects how the unit rents and appraises.
Underestimating construction access for a detached build on a tight, no-alley lot — access constraints can quietly add significant cost or rule out detached entirely.
Comparing online averages instead of scoped numbers for your actual structure. Generic calculators almost always flatter the conversion by omitting the upgrade line items.
The reliable way through this decision is not a blog estimate — it is putting eyes on your actual garage and lot. During a free in-home estimate, Iron Crest Remodel evaluates the existing slab, footing, ceiling height, egress feasibility, and electrical service so the garage-conversion number is real rather than an average. We assess buildable area, setbacks, coverage, and construction access for a detached alternative on the same lot, and we confirm the applicable rules with the current City of Boise or Ada County requirements for your parcel rather than relying on prior projects. You receive an honest side-by-side so you can choose with real numbers.
Iron Crest Remodeling Group LLC (Iron Crest Remodel) is licensed and insured in Idaho, RCE-6681702, and every project is backed by our 5-year workmanship warranty. We serve Boise and the Treasure Valley Monday through Friday, 7 AM to 6 PM. Reach our team at (208) 779-5551, request a free estimate, or contact us to start the conversation. For deeper background, browse our remodeling guides.
Which ADU type produces higher rental income in Boise?
A detached ADU almost always generates higher rental income because it can be built larger, offers a completely separate entrance and address, and provides the privacy that tenants value most. In central Boise, the North End, and the Bench, a well-finished one-bedroom detached ADU tends to command a meaningful premium over a comparable garage conversion, because the garage conversion is usually smaller, has a lower finished ceiling, and sits close to the main house. Detached units also tend to perform better as furnished mid-term or short-term rentals because guests perceive them as private guest houses rather than converted spaces. We do not quote specific rents here because Boise rental rates move with the market, season, and exact location — ask a local property manager for current comparable rents before you model your return.
Is a garage conversion always cheaper than a detached ADU in Boise?
Usually, but not always, and the gap is smaller than most homeowners expect. A garage conversion is cheaper when you can genuinely reuse the existing slab, walls, and roof, and when utilities are close. The savings disappear quickly when the slab is uninsulated and below code for habitable space, when the foundation has no frost footing for a heated structure, when ceiling height is too low once you add insulation and finished ceiling, when egress windows must be cut into masonry or concrete, or when the existing electrical service cannot carry a second dwelling. A detached ADU costs more on paper but the budget is more predictable because you are building new and controlling every assembly from the ground up. We scope both options against your actual structure during the in-home estimate so you are comparing real numbers, not averages.
What Boise and Ada County zoning rules affect this decision?
ADU regulations are set by the jurisdiction your property sits in — the City of Boise for most in-city lots, or Ada County Development Services for unincorporated areas — and they change over time. The rules that most often shift the detached-versus-conversion decision are minimum lot size, required setbacks, maximum ADU size, total lot coverage limits, owner-occupancy requirements, off-street parking requirements, and whether more than one ADU is allowed on a lot. A garage conversion that reuses an existing footprint can sometimes be permitted on a lot that would not support a new detached structure under current setback or coverage limits. None of these specifics should be treated as fixed: always verify the current City of Boise or Ada County code for your exact parcel and zoning district before you design, because adopted code and interpretations are updated periodically.
How long does the permit process take for each ADU type in Boise?
Garage conversions generally move through plan review faster because the existing structure already has an approved foundation, roof, and exterior shell, so the review focuses on change of use, egress, energy code, and the trade work. Detached ADUs require full plans — site plan with setbacks and lot coverage, structural details, and separate trade permits — so review takes longer and fees are higher. Treat any week or fee figure as an estimate only: City of Boise and Ada County review queues, submittal completeness, and fee schedules change, so confirm current timelines and costs with the jurisdiction when you apply.
Will I lose my parking if I convert my garage to an ADU?
Yes. Converting the garage permanently removes that covered parking, which usually pushes household vehicles to the driveway or street. Whether you must replace that parking off-street depends on current City of Boise or Ada County parking requirements for ADUs in your zone, and those requirements change — verify the rule that applies to your parcel before you design. Beyond code, there is a livability and resale cost: Boise winters bring freezing temperatures and snow, and many local buyers expect garage parking. Some homeowners add a carport or expand the driveway to soften the loss, which adds to the budget and rarely fully replaces an enclosed garage.
Can I build a detached ADU if I already have a detached garage on my lot?
Sometimes. Your existing house and detached garage already count toward the maximum lot coverage allowed in your zoning district, so the question is whether the lot has room for the ADU footprint while staying under the current coverage limit and meeting all setbacks. On tighter lots, homeowners sometimes remove an aging detached garage and build the detached ADU in its place, which simplifies the coverage math and can shorten utility runs. Because coverage limits, setbacks, and one-ADU-per-lot rules vary by jurisdiction and are updated over time, verify the current City of Boise or Ada County requirements for your parcel — we evaluate lot dimensions, zoning, and existing structures during the site consultation.
Does a garage conversion or detached ADU add more property value in Boise?
Both add value, but appraisers and buyers tend to treat a true detached dwelling more favorably because it lives like an independent home and, when separately metered, is easier to value as an income unit. A garage conversion adds value too, but some appraisers discount it for the lost garage, and the smaller footprint caps the value it can add. The honest answer is that value depends on quality of finish, whether the unit is legal and permitted, separate metering, and neighborhood comparables — an unpermitted conversion can actually complicate a sale. We do not publish dollar figures because appraised value is parcel- and market-specific; a local appraiser or agent can give you current comparables.
How do utilities differ between a detached ADU and a garage conversion?
A detached ADU usually needs new trenched runs for water, sewer, electrical, and often gas from the main house or the street, plus its own electrical sub-panel or a dedicated panel, and sometimes a separate water or sewer tap depending on the utility provider and jurisdiction. A garage conversion ties into existing house systems with much shorter runs, which is cheaper, but the existing electrical service — common in older Boise homes — may need an upgrade to carry a second dwelling, and the existing water heater and HVAC may be undersized. Separate metering is the other major variable: it improves how the unit rents and appraises but adds cost and utility-company coordination. Tap fees and metering rules are set by the utility and jurisdiction and change, so confirm current requirements during design.
Which is better for housing aging parents or adult children?
For multigenerational living, a garage conversion is often the practical winner when the goal is proximity, a faster timeline, and a lower budget — the unit is close to the main house, single-level, and quicker to occupy. A detached ADU is the stronger choice when the family member wants real independence and privacy, when a one-level aging-in-place layout with wider doorways and a curbless shower matters, or when you expect to convert the unit to a rental later. Many Boise families choose based on the next decade, not just today: detached preserves the most future optionality, a conversion gets a parent moved in sooner.
What are the most common mistakes Boise homeowners make with this decision?
The biggest mistake is assuming a garage conversion will be cheap before anyone has checked the slab thickness, frost footing, ceiling height, egress, and electrical service — those upgrades routinely erase the expected savings. The second is ignoring parking loss and its resale impact in a market where buyers expect a garage. The third is treating zoning as static: setbacks, coverage, owner-occupancy, parking, and max-size rules change, and a design that pencils today may not match the code in effect when you submit — always verify current code. The fourth is building unpermitted to save time, which damages value and complicates a future sale. The fifth is sizing a detached ADU to the budget rather than to the rent and resale ceiling, leaving long-term income on the table.
Can I convert an attached garage, and how is that different from a detached garage?
Both can be converted, but they behave differently. An attached garage shares one or more walls with the house, so conversion is structurally simpler and utilities are closest, but you inherit shared-wall sound transmission and less tenant privacy, and many attached-garage conversions function more like an attached ADU or in-law suite than a fully independent unit. A detached garage conversion produces a more private, independent unit similar to a small detached ADU, but utility runs are longer and the structure was often built lighter than a heated dwelling requires. The right comparison for an attached garage is frequently a detached ADU versus an attached ADU rather than a like-for-like garage conversion — we walk through both during the estimate.
Does lot size or alley access change which ADU type wins?
Yes, site constraints often decide it. A garage conversion has essentially no new footprint, so it can work on small Boise Bench or older interior lots where a new detached structure would violate setbacks or coverage. A detached ADU needs enough buildable area and realistic construction access — a path to bring materials and equipment to the backyard. North End and Bench lots with alley access are strong candidates for detached units because the alley provides a private entrance, separate address feel, and easier construction staging, which is why those neighborhoods see strong detached-ADU interest. On a landlocked deep lot with no alley and a narrow side yard, a conversion may be the only feasible option. Confirm setbacks, access easements, and coverage for your specific parcel and current code.
How do I decide between the two for my Boise property?
Start with three questions: what is your real budget ceiling, what does your lot and zoning actually allow today, and is your goal maximum long-term income and resale or the fastest, lowest-cost path to a usable unit. If the answers point to income, privacy, and resale and the lot supports it, detached usually wins. If they point to budget, speed, proximity, and a structurally sound existing garage you do not need for parking, a conversion usually wins. Because the deciding factors are parcel-specific and code changes over time, the most reliable next step is a free in-home estimate where Iron Crest Remodel scopes both paths against your actual structure, lot, and current jurisdiction requirements.
This comparison is one piece of our ADU resource library for Boise and Treasure Valley homeowners. Explore the guides below to go deeper on the path that fits your project.
The following government agencies, industry organizations, and official resources provide additional information relevant to your remodeling project.
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