Washington residential & ADU permit data

Washington's House Bill 1337 (2023) is one of the most prescriptive statewide ADU laws in the country. Inside urban growth areas, cities and counties must allow at least two accessory dwelling units on any lot that permits a single-family home — attached, detached, or a mix — and must permit ADUs of at least 1,000 square feet. The law took effect in July 2023, but each jurisdiction has to fold the standards into its code within six months of its next periodic comprehensive-plan update, so the bulk of larger Puget Sound cities phased in compliance through 2025.

The statute removes the usual local barriers in unusual detail. Owner-occupancy requirements are banned outright. Off-street parking cannot be required within a half-mile walking distance of a major transit stop, and elsewhere is capped at one space per unit on lots under 6,000 square feet and two per unit on larger lots. Impact fees on an ADU are limited to 50% of what the jurisdiction charges on the primary house, and cities must allow ADUs to be sold separately as condominium units. Cities keep authority over objective design and building-code standards but not over whether to allow the units.

HB 1337 passed alongside HB 1110, the state's 'missing middle' law, which ends single-family-only zoning in larger cities by requiring two-to-six units per lot scaled by city size and transit access. Together they mean a typical urban Washington lot can carry a middle-housing building and ADUs, sharply increasing the legal density a single parcel can support.

Washington's framework is more detailed than Oregon's earlier ADU floor and, in one specific respect, broader than California's: it guarantees two ADUs per lot statewide inside urban growth areas, where California still leads on ministerial multifamily ADUs and lot splits. Compared with the big-city-only mandates in Colorado and Arizona, Washington reaches all jurisdictions within urban growth areas. Igni sources live Seattle permit data from the city's license-free open-data portal, with Tacoma and Bellevue feeds, so new ADU and middle-housing permits appear as the reforms take hold.

This is an informational summary, not legal advice; each jurisdiction's adopted code controls the precise standards, so verify the current local rules before relying on it.