ADU rules in Washington
Representative cities: Seattle, Spokane.
ADU permit rule
Permit requiredGenerally allowedcity authority
Fee basis
Valuation-based; Seattle SDCI issue fees scale with project value (no flat state fee).
Typical timeline
Seattle pre-approved DADU plans ~2–6 weeks; custom plans longer.
Setback
Seattle: 5 ft rear (0 ft where lot abuts an alley); varies by city/zone.
Area threshold
Statewide HB 1337 (eff. 2025) allows ≥1 ADU per lot; Seattle allows up to 2 ADUs.
Exempt if
None — a building permit is always required for an ADU.
Go / no-go
allowed
Washington HB 1337 (2025) mandates ADUs; Seattle also allows two ADUs per lot. Owner-occupancy and extra parking are not required in Seattle.
ADU official sources: Seattle SDCI — Accessory Dwelling Units · WA State Building Code Council — 2021 IRC (WAC 51-51)
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Frequently asked questions
Do I need a permit to build an ADU in Washington?
Per Washington rules, an ADU is "yes" for a permit and the authority is "city". Washington HB 1337 (2025) mandates ADUs; Seattle also allows two ADUs per lot. Owner-occupancy and extra parking are not required in Seattle. Always confirm with the issuing building department.
How big can an ADU be in Washington?
Area threshold: Statewide HB 1337 (eff. 2025) allows ≥1 ADU per lot; Seattle allows up to 2 ADUs.
What are the ADU setbacks in Washington?
Setback: Seattle: 5 ft rear (0 ft where lot abuts an alley); varies by city/zone.
How much does an ADU permit cost in Washington?
Fee basis: Valuation-based; Seattle SDCI issue fees scale with project value (no flat state fee).
State-wide sources: WA State Building Code Council (home) · WA 2021 IRC adoption (WAC 51-51) · Seattle SDCI — Common Projects · City of Spokane — Residential Building Review
Plan the ADU numbers for Washington
ADU build cost calculator
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Rental ROI and payback
Model rent, cash flow and how long the build takes to pay for itself.
Financing comparison
HELOC vs construction loan vs home equity loan vs cash, on example rates.
Does your ADU need a permit?
Run the go / no-go checker with your own project details.