AGrannio

Can You Build an ADU in Seattle?

Yes — Seattle is governed by Washington's statewide ADU law, which limits what the city can restrict. A 700 sq ft detached ADU here runs about $189,500–$310,500.

Construction in Seattle runs about 12% above the Washington average, so budget toward the higher end of these ranges.

Detached ADU (700 sq ft)

$189,500–$310,500

Garage conversion (400 sq ft)

$59,000–$108,500

Statewide protection

Yes

What applies in Seattle

Washington's statewide standard (RCW 36.70A.681 (HB 1337, 2023)) is the starting point, but parcel-level rules, fees and design review are set locally. Confirm those details with Seattle before design.

Estimate your Seattle ADU cost

Your property

ADU type

Estimated cost in Washington

$189,500 – $310,500

≈ $271–$444/sq ft turnkey · midpoint $250,000

$170,000
Construction
$45,000
Design & permits
$35,000
Site work
Good news — building an ADU in Washington is very likely allowed.
  • ✓Size within the guaranteed minimum. Your 700 sq ft ADU is at or under the 1000 sq ft your city must allow regardless of other rules.
  • ✓5 ft setback. State law caps the side and rear setback at 5 ft, so even a tight lot usually works.
  • ✓No extra parking likely required. No more than 1 space per ADU; no parking required within 1/2 mile of major transit.
  • ✓No owner-occupancy requirement. You can rent out both the main home and the ADU — you do not have to live on the property.
  • ✓Lot has room. A 700 sq ft ADU uses about 12% of your 6,000 sq ft lot — comfortably within typical coverage limits.

Rule basis: RCW 36.70A.681 (HB 1337, 2023)

If you rent it out

$1,175–$1,925

est. rent/mo

7.4%

gross yield

14.5 yrs

payback

~$17,298/yr after a 7% vacancy allowance, before financing & maintenance.

Financed over 20 yrs at 7.5%, the build is about $2,014/mo — vs ~$1,550/mo in estimated rent.

Get your detailed feasibility report →

Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.

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Other Washington cities

SpokaneTacomaVancouverBellevueEverettKentRentonOlympia

Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.