ADU / Guest House cost
What an ADU involves, what it runs in each state we build in, and the metro-by-metro guides with live pricing.
What the project involves
An accessory dwelling unit (ADU) is a complete second home on your lot — a detached backyard cottage, a garage conversion or a basement apartment with its own kitchen, bath and entry. Detached units carry their own foundation, roof and utility runs; conversions reuse the shell and cost less.
ADUs are priced per square foot like other whole-structure work, with a strong size factor because small buildings pack kitchens, baths and mechanical rooms into few square feet. Local zoning, parking and utility rules shape what can be built and what it costs to connect.
Across our four states
Live planning ranges from the current pricing version. Land, design and permit fees, utility connections and unusual soils are priced separately.
| State | Priced at | ADU, standard finish |
|---|---|---|
| Washington | Seattle · 800 sq ft (exact) | $355K – $485K |
| Idaho | Boise · 800 sq ft (exact) | $248K – $335K |
| Montana | Billings · 800 sq ft (exact) | $216K – $290K |
| Arizona | Scottsdale · 800 sq ft (exact) | $325K – $455K |
ADU / Guest House cost by metro
Metro guides for this project type are being prepared — use the calculator for a live range in your ZIP.
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