Boise · Treasure Valley · Garage Conversion ADU
Turn an existing detached or attached garage into a permitted, code-compliant ADU. Typically $60k–$140k turnkey, 3–4 months contract-to-keys, and significantly cheaper than a ground-up build.
A garage conversion turns an existing permitted garage — usually detached, sometimes attached — into a fully permitted accessory dwelling unit. The foundation, exterior walls, and roof stay. Insulation, framing, electrical, plumbing, mechanical, finishes, and code upgrades all get added. The end product is a real, rentable, appraisable ADU. In Boise, garage conversions typically run $60k–$140k turnkey and finish 3–4 months from contract — roughly 30–50% cheaper and 30–60 days faster than a ground-up new build. The trade-off: you're locked to the garage's existing footprint, so layouts are tighter and a 2-bedroom plan usually isn't possible. For studios and 1-bedrooms on a Boise lot that already has the right garage, conversions are usually the highest-ROI ADU configuration available.
The honest side-by-side. Conversion wins on cost and speed; new build wins on size flexibility and resale ceiling.
| Factor | Conversion | New build | Why |
|---|---|---|---|
| Cost | $60k–$140k typical | $115k–$345k turnkey | Conversion reuses foundation, walls, and roof — that's where the savings come from. |
| Timeline | 3–4 months | 5–7 months | Less site work, less framing. Permitting still 3–5 weeks even on conversions. |
| Square footage flexibility | Locked to existing footprint | Up to 900 sqft per Boise code (2023 update) | If you want a 2BR Kestrel layout (695 sqft), a typical 2-car garage at ~440 sqft won't get you there. |
| Foundation reuse | Yes — usually | New foundation required | Existing garage slabs often need a vapor barrier retrofit and edge insulation, but the slab itself is typically reusable. |
Not every garage is a candidate. The four configurations below are the ones we see convert cleanly in Boise — fewest surprises, tightest budget range, fastest permits.
The most common Boise conversion. A typical 20'x22' or 22'x22' detached garage gives you ~440-484 sqft — enough for a 1-bedroom layout with full kitchen, bathroom, and a real living space. Existing concrete slab, walls, and roof all stay.
Older Boise neighborhoods (North End, East End, parts of Boise Bench) have alley-loaded detached garages that are perfect for ADU conversion. Separate access from the alley, no parking impact on the front street, and typically already set back from the main house.
An attached garage that already has its own exterior door and a path away from the main house can convert cleanly — but fire-separation upgrades, dedicated HVAC, and a sound wall between the units add cost. Plan for the high end of the range.
Some Boise lots have a permitted shop, carriage house, or oversized accessory building that's eligible for conversion. If the foundation is sound and the structure was originally built to permit, it's usually a candidate — we verify with the city before committing.
A converted garage has to meet the same residential code as a ground-up ADU. These are the five upgrades the City of Boise looks for at every conversion permit submittal.
Garages aren't typically built to residential energy code. Conversion means R-21 walls, R-49 ceiling, and full air sealing — usually new framing inside the existing walls or batts plus rigid foam.
Each bedroom needs an egress window meeting Idaho residential code (5.7 sqft net clear opening, 24" minimum height, 20" minimum width, sill no higher than 44" off the floor). Cutting a new opening in an existing garage wall is straightforward but adds cost.
Boise residential code requires a 7'6" minimum ceiling in living spaces. Most garages built in the last 30 years already clear this. Older detached garages occasionally don't — we measure during the site visit.
Boise allows shared utility services with the main house. You can install a separate electrical panel, water meter, or gas service for cleaner billing on a rental, but it's not required. Sub-metering is the cheap middle path.
Attached garage conversions need a 1-hour fire-rated wall and ceiling assembly between the new ADU and the main house. This typically means 5/8" Type X drywall on both sides of the assembly and rated doors — not expensive but mandatory.
Turnkey, fixed-price ranges. Includes insulation, framing, MEP, drywall, finishes, paint, fixtures, and permits. Excludes furniture, premium appliance upgrades, and exterior landscaping.
| Garage type | Size | Price range | Notes |
|---|---|---|---|
| Small 2-car detached garage | ~440 sqft | $60k–$90k | Cleanest conversion. Studio or open 1BR layout. Existing slab and roof in good shape. |
| Large 2-car detached garage | ~600 sqft | $80k–$120k | Room for a real 1BR with separate bedroom. Most common premium conversion in Boise. |
| Small detached garage / shop | ~360 sqft | $55k–$85k | Single-car or compact shop. Studio layout. Most lot-flexible option for tight properties. |
| Attached garage conversion | ~440 sqft | $80k–$140k | Higher end of range due to fire separation, sound wall, and dedicated HVAC requirements. |
Yes — typically 30-50% less than a comparable new-build ADU in Boise. A 2-car garage conversion runs $60k-$120k turnkey vs $170k for a 491 sqft Kingfisher new build. The savings come from reusing the foundation, exterior walls, and roof. Where you don't save: insulation, MEP, finishes, and permits — those are the same.
Yes, always. Converting a garage to permitted living space requires a building permit, electrical permit, plumbing permit, and mechanical permit at minimum. The City of Boise also requires a planning review to confirm your lot allows a second dwelling unit. Unpermitted conversions create resale problems, insurance problems, and sometimes lender problems.
Almost never. Most Boise garages have either no heat or a basic gas unit heater that doesn't meet residential code. Conversion typically means a new mini-split heat pump system (one or two indoor heads, an outdoor unit) — efficient, code-compliant, and around $7k-$11k installed. Sometimes a shared furnace branch from the main house works, but mini-splits are usually cleaner.
Detached. By a wide margin. Attached garage conversions require a 1-hour fire-rated separation, a sound wall, dedicated egress, and often a separate HVAC system to avoid air-sharing with the main house. Detached garages skip most of that. Detached is also typically a faster permit because the city already treats it as a separate structure.
Idaho residential code (the 2018 IECC with Boise amendments) requires R-21 walls, R-49 attic, R-30 floor over unheated space, and full air sealing. Most existing garages have R-0 to R-13 walls and R-0 ceilings. Plan for new framing inside the existing walls (or rigid foam over the exterior), full attic insulation, and a slab edge insulation retrofit if the slab abuts conditioned space.
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Start with a free lot check covering structural fit, code upgrades, and a recommended path. Or call (208) 297-2036 to talk it through.
