Detached ADU vs Garage Conversion in Boise
A side-by-side comparison of the two most popular ADU options for Boise homeowners — covering costs, timelines, permits, rental income, property value impact, and the best use case for each on your lot.
Boise homeowners exploring accessory dwelling units face one fundamental decision early in the planning process: build a detached ADU from the ground up in the backyard, or convert an existing garage into livable space. Both options create a legal, code-compliant dwelling unit on your property, but they differ dramatically in cost, timeline, complexity, and long-term return. Choosing the wrong path wastes months and tens of thousands of dollars.
This comparison guide is built from our experience designing and building both ADU types across the Treasure Valley — from compact garage conversions on the Boise Bench to full detached cottages in the North End and Eagle foothills. We break down every factor that matters so you can make the right investment decision for your lot, your budget, and your goals.
If you are building for maximum rental income and long-term property value, a detached ADU is typically the stronger play. If you need an ADU faster and at lower cost and are willing to trade parking and some privacy, a garage conversion delivers results in half the time and at roughly half the budget.
Cost is the most decisive factor for most Boise homeowners. A detached ADU is a ground-up construction project that requires a new foundation, framing, roofing, siding, utility connections, and full interior buildout. A garage conversion reuses the existing foundation, walls, and roof structure, which eliminates roughly 30 to 40 percent of the construction scope.
Detached ADU
$150,000 – $250,000+
New foundation (slab or crawlspace), full framing, roofing, exterior finishes, utility trenching and connections, insulation, drywall, flooring, kitchen, bathroom, HVAC system, and electrical panel. Cost per square foot in Boise ranges from $300 to $400+. Site access, slope, and utility distance from the main house are the primary variables that push costs toward the upper end.
Garage Conversion
$80,000 – $150,000
Reuses existing foundation, walls, and roof. Budget covers insulation, drywall, flooring, window and door modifications, plumbing for a bathroom and kitchenette, electrical upgrades, HVAC, and interior finishes. Cost per square foot typically falls between $150 and $300. Garage door removal and wall infill, foundation moisture sealing, and subfloor installation over the concrete slab are unique line items for conversions.
Your ADU type determines both the available square footage and how quickly you can have a tenant or family member moved in. These differences cascade into layout options, bedroom count, and livability.
Size & Layout
Detached ADUs in Boise are typically built between 400 and 900 square feet with full flexibility in layout — studio, one-bedroom, or two-bedroom configurations with dedicated living areas, full kitchens, and bathrooms. Garage conversions are constrained by the existing garage footprint, which is usually 400 to 576 square feet for a standard two-car garage. This limits most conversions to a studio or one-bedroom layout. Ceiling height is another constraint: many Boise garages have 8-foot ceilings that feel tight once insulation and finished ceiling materials are added, whereas detached builds allow 9-foot or vaulted ceilings for a more spacious feel.
Construction Timeline
A detached ADU takes 6 to 10 months from permit approval through certificate of occupancy. This includes foundation work (2–3 weeks), framing (2–4 weeks), roofing and exterior (2–3 weeks), rough-in trades (2–3 weeks), insulation and drywall (2 weeks), and finish work including kitchen and bath (4–6 weeks). A garage conversion takes 3 to 5 months because it skips foundation, framing, and roofing entirely. The construction sequence moves directly to insulation, framing interior partitions, plumbing and electrical rough-in, drywall, and finish work. The shorter timeline means you start generating rental income or housing a family member months sooner.
The regulatory and practical differences between these two ADU types extend well beyond cost and schedule. Permit complexity, sound separation, and utility infrastructure each have a meaningful impact on your project experience and long-term satisfaction.
Permit Complexity in Boise
Detached ADUs require full new-construction permits from the City of Boise: architectural plans, structural engineering, site plans showing setbacks and lot coverage, and separate trade permits for plumbing, electrical, mechanical, and potentially sewer and water tap fees. Plan review runs 6 to 12 weeks. Garage conversions require a change-of-use permit and trade permits, but the existing approved structure simplifies the review process significantly. Plan review for conversions typically takes 4 to 8 weeks. Both types must comply with Boise's ADU zoning requirements, including minimum lot size, setback distances, and maximum lot coverage ratios.
Privacy & Sound Separation
A detached ADU provides complete physical separation from the main house with its own walls, roof, and airspace gap. This delivers superior sound isolation and visual privacy for both the homeowner and the tenant. Garage conversions share one or more walls with the main house (attached garages) or sit very close to it, which means sound transmission through shared walls, footstep noise, and reduced privacy. For long-term rental use, this difference is significant — tenants pay more for and stay longer in units that feel like independent homes rather than converted garage spaces.
Utility Connections
Detached ADUs require trenching for water, sewer, electrical, and gas lines from the main house or street to the new structure. In Boise, utility connection costs range from $8,000 to $20,000 depending on the distance and whether new taps are required by Boise City Public Works or Suez Water Idaho. The ADU also needs its own electrical sub-panel or dedicated panel. Garage conversions tie into the existing house utilities with much shorter runs, keeping connection costs between $3,000 and $8,000. However, the main house electrical panel may need an upgrade to handle the additional load, especially in older Boise homes with 100-amp service that needs to be upgraded to 200 amps.
Both ADU types generate rental income and increase property value, but the magnitude differs. Understanding the financial return profile of each option helps you align your ADU investment with your financial goals.
Detached ADU
Monthly rental income: $1,200–$1,800 (long-term) or $100–$175/night (short-term)
Property value increase: $100,000–$200,000 in appraised value
Perceived as independent dwelling — higher tenant demand and lower vacancy
Appraisers treat detached ADUs more favorably due to separate utility metering
Stronger Airbnb/short-term rental performance due to guest house appeal
Garage Conversion
Monthly rental income: $900–$1,400 (long-term) or $70–$130/night (short-term)
Property value increase: $60,000–$120,000 in appraised value
Faster payback period due to lower construction cost despite lower rent
Some appraisers discount value due to loss of garage parking
Better suited for family use (aging parents, adult children) than premium rentals
The parking question is one of the most overlooked factors in the detached vs. garage conversion decision, and it has real implications for daily life, resale value, and neighborhood relations in Boise.
Detached ADU
Your garage stays intact. All existing covered parking is preserved. The ADU sits in the backyard without affecting the driveway or street parking. This is a major advantage for Boise homes with two-car garages, especially in neighborhoods where street parking is limited or HOA rules restrict driveway vehicles.
Garage Conversion
Converting the garage eliminates covered parking entirely. All vehicles move to the driveway or street. In Boise winters, this means scraping ice and clearing snow from vehicles daily. It also reduces curb appeal for future buyers who expect garage parking. Some homeowners add a carport ($5,000–$15,000) to partially offset the loss, but it never fully replaces the functionality of a garage.
This table summarizes every major factor so you can compare both ADU options at a glance and identify which aligns with your priorities.
| Factor | Detached ADU | Garage Conversion |
|---|---|---|
| Total Cost | $150K–$250K+ | $80K–$150K |
| Cost per Sq Ft | $300–$400+ | $150–$300 |
| Construction Timeline | 6–10 months | 3–5 months |
| Typical Size | 400–900 sq ft | 400–576 sq ft |
| Bedroom Options | Studio, 1BR, or 2BR | Studio or 1BR |
| Permit Complexity | Full new construction | Change-of-use + trades |
| Plan Review Time | 6–12 weeks | 4–8 weeks |
| Monthly Rent (Long-Term) | $1,200–$1,800 | $900–$1,400 |
| Property Value Increase | $100K–$200K | $60K–$120K |
| Parking Impact | None — garage preserved | Garage parking eliminated |
| Privacy & Sound | Full separation | Shared wall(s) or close proximity |
| Utility Connections | New trenching required | Tie into existing house systems |
| Utility Cost | $8K–$20K | $3K–$8K |
| Best For | Max rental income, Airbnb | Budget-friendly, family use |
The right choice depends on your goals, budget, lot, and timeline. Here is how to match each ADU type to common Boise homeowner scenarios.
Choose a Detached ADU If You…
Want to maximize long-term rental income with premium rent rates
Plan to use it as an Airbnb or short-term rental near downtown Boise
Need a two-bedroom layout or more than 576 square feet
Value complete privacy and sound separation from tenants
Want to preserve your garage for parking and storage
Have a lot large enough to meet Boise setback and coverage requirements
Are building for resale value and the strongest possible appraisal impact
Choose a Garage Conversion If You…
Need to keep your total budget under $150,000
Want the ADU livable in 3 to 5 months rather than 6 to 10
Are housing an aging parent or adult child who benefits from proximity
Have a solid, well-maintained garage with a good foundation and roof
Do not rely on your garage for daily vehicle parking
Want a simpler permit process with faster city approval
Prefer the lower financial risk of a smaller initial investment
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. A well-finished one-bedroom detached ADU in central Boise or the North End can rent for $1,200 to $1,800 per month, while a comparable garage conversion typically rents for $900 to $1,400 per month due to its smaller footprint and shared-wall proximity to the main house. Detached units also command premium short-term rental rates on platforms like Airbnb because guests perceive them as private guest houses rather than converted spaces. Over a 10-year period, the higher rental income from a detached ADU often recovers the additional construction cost compared to a garage conversion.
How long does the permit process take for each ADU type in Boise?
Garage conversions in Boise generally have a simpler and faster permit process because the existing structure already has an approved foundation, roof, and exterior walls. Plan review for a garage conversion typically takes 4 to 8 weeks through the City of Boise Planning and Development Services, and the permit fees are lower since less new construction is involved. Detached ADU permits require full architectural plans, site plans showing setbacks and lot coverage, and separate trade permits for foundation, framing, plumbing, electrical, and mechanical work. Plan review for a new detached structure takes 6 to 12 weeks, and total permit fees typically run $3,000 to $8,000. Both ADU types must comply with Boise zoning code requirements for lot size, setbacks, and maximum lot coverage, but the detached build triggers additional review for site grading, utility routing, and sometimes stormwater management.
Will I lose my parking if I convert my garage to an ADU?
Yes, converting your garage eliminates that covered parking space permanently. In most Boise neighborhoods, this means you will need to park in the driveway or on the street. The City of Boise does not currently require replacement off-street parking for ADU projects in most residential zones, but losing garage parking can be a significant lifestyle change, especially during Idaho winters when temperatures regularly drop below freezing and snow accumulates on vehicles. Some homeowners offset this by adding a carport or expanding their driveway, which adds $5,000 to $15,000 to the project budget. Before committing to a garage conversion, evaluate whether your driveway can accommodate your household vehicles and consider the impact on resale appeal, since many Boise buyers prioritize garage parking.
What is the ROI difference between a detached ADU and a garage conversion in Boise?
Both ADU types deliver strong returns in Boise, but the math works differently for each. A garage conversion typically costs $80,000 to $150,000 and adds $60,000 to $120,000 in appraised property value, representing a 70 to 85 percent cost-to-value recovery at sale plus ongoing rental income during ownership. A detached ADU costs $150,000 to $250,000 or more and adds $100,000 to $200,000 in property value, also recovering roughly 70 to 85 percent of construction cost at appraisal. However, the detached unit generates higher monthly rental income, so the payback period through rental revenue is often comparable despite the higher upfront cost. In Boise neighborhoods with strong rental demand like the North End, Downtown, and Boise Bench, a detached ADU can break even in 8 to 12 years through rental income alone. A garage conversion typically breaks even in 6 to 10 years due to the lower initial investment.
Can I build a detached ADU if I already have a detached garage on my lot?
It depends on your lot size, existing lot coverage, and Boise zoning district. The City of Boise limits total lot coverage for all structures combined, and your existing house plus detached garage already count toward that limit. In R-1C zones, the maximum lot coverage is 40 percent, while R-2D and R-1M zones allow higher coverage. If your lot is large enough to accommodate the ADU footprint while staying under the coverage limit and meeting all setback requirements, you can build a detached ADU in addition to your existing garage. In some cases, homeowners choose to demolish an aging detached garage and build a new detached ADU in its footprint, which simplifies the lot coverage calculation and avoids utility routing challenges. Iron Crest Remodel evaluates your lot dimensions, zoning designation, and existing structures during our initial site consultation to determine exactly what is buildable on your property.
This comparison is one piece of our comprehensive ADU resource library for Boise homeowners. Explore the guides below to dive deeper into the specific ADU type 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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