Cost guide
ADU Cost in Boise, Idaho
Sourced 2026 ranges, then the local factors that move a Treasure Valley ADU budget up or down.
Price ranges at a glance
These are national figures from Angi's 2026 ADU cost data, not our price list (Angi). Your written estimate is based on your lot after a site visit.
| Item | Typical range |
|---|---|
| National average, all ADU types | About $180,000 |
| Most homeowners spend | $60,000 – $285,000 |
| Garage or basement conversion | $60,000 – $150,000 |
| Detached new construction | $110,000 – $285,000 |
| Per square foot | $150 – $300 |
| Building permit (national average) | About $1,350 |
What moves the price in the Treasure Valley
Utilities
Water, sewer and power have to reach the unit. A conversion close to the main house's plumbing costs far less than a detached cottage at the back of a deep lot that needs a long trench and a new sewer connection. A main electrical panel upgrade is common when the house has an older or smaller panel. Under Idaho Code § 67-6541, cities over 10,000 people can't charge ADU impact or utility connection fees higher than for other single-family homes after February 1, 2027 (Idaho Legislature).
Design
The City of Boise says a custom ADU design can cost upwards of $10,000. Its free pre-approved plans remove most of that for eligible lots, though site plans, utilities and impact fees still apply.
Size and shape
Smaller units cost more per square foot because the kitchen and bathroom, the most expensive rooms, are a bigger share of the total. A simple rectangle with one roof line costs less than a unit with jogs, dormers or a second story.
Site conditions
Slope, access for equipment, trees to protect, irrigation laterals and easements, and floodplain requirements near the Boise River all add cost and time.
Finish level
Cabinets, counters, flooring, windows and appliances can swing a budget by tens of thousands of dollars. Durable mid-range finishes make sense for a rental; an ADU for family may justify more.
Ways to keep the budget in check
- Use a pre-approved plan if your Boise lot qualifies.
- Place the unit close to utilities to shorten trenches.
- Keep the shape simple: one story, one roof line, stacked plumbing.
- Convert before you build if a sound garage or basement is available.
- Compare bids on the same scope, including utilities, permits, impact fees and site restoration, and check each contractor's Idaho registration with the Idaho Division of Occupational and Professional Licenses.
What a detached ADU estimate includes
- Design and engineering: plans, structural engineering where needed, or a pre-approved plan.
- Permits and fees: planning and building permits, plan review, and impact and utility connection fees.
- Site work: clearing, grading, excavation, and restoration of the yard afterward.
- Utilities: water, sewer and electrical runs, and any panel upgrade.
- Structure: foundation, framing, roof, windows, doors and siding.
- Systems and finishes: plumbing, electrical, heat pump, insulation, drywall, flooring, cabinets, counters and fixtures.
Paying for an ADU
Homeowners commonly fund ADUs with cash, a home equity line of credit, a cash-out refinance or a construction loan. Some lenders consider expected rental income. Talk to a lender early: the written, itemized estimate we provide is what they'll ask for.
Does an ADU pay for itself?
Many Boise-area owners rent their ADU long term, and the rent can cover a large share of the construction loan payment. An ADU can also add resale value and flexibility for a growing or aging family. The math depends on your financing, the unit's size and local rents, so we give you the build cost in writing to plug into your own numbers.
How our estimates work
After a lot check and site visit, you get a written estimate that separates design, permits and fees, site work, utilities, foundation, structure, mechanical and finishes, so you can see where the money goes and make trade-offs. See detached ADUs and garage conversions for what each includes.
FAQ
ADU cost questions
Do Boise's pre-approved plans make an ADU cheap?
They save design fees, which the city says can run upwards of $10,000, but construction, utilities, site work and impact fees still make up most of the cost.
Is a garage conversion always cheaper than a detached ADU?
Usually, but not if the slab, framing or roof need major work. We price both when it's close.
What is the cheapest way to add an ADU?
Converting a sound garage or basement close to existing plumbing. In Boise, using a free pre-approved plan also saves design fees for a detached unit.
Are utility hookups included in ADU cost estimates?
They should be. Ask every builder to list water, sewer and electrical work and any impact fees separately.
How much does a 600 square foot ADU cost?
At Angi's $150 to $300 per square foot, roughly $90,000 to $180,000, before site-specific utilities and fees.