Grannio

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 HB 1337 (2023) requires cities and counties in urban growth areas to allow two ADUs per lot, of at least 1,000 sq ft each, in single-family zones. Owner-occupancy cannot be required and parking minimums are sharply limited near transit.

Because Washington has a statewide ADU law (RCW 36.70A.681 (HB 1337, 2023)), Seattle must follow these minimums even where its older zoning code is stricter. Confirm fees and design standards with the Seattle planning department.

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.