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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Model this ADU's construction cost by type, size & finish.
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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).