ADU Design in Wapato, WA

ADU Design in Washington: what governs your project

Washington's HB 1337 (2023) requires cities and counties planning under the Growth Management Act to allow at least two ADUs per lot in urban growth areas, by right. The law preempts local owner-occupancy mandates, off-street parking requirements beyond the primary home's, and design standards stricter than those for houses.

Energy code

Impact fees for ADUs are capped by state law at a proportional share of the primary home's fees. Local building departments in Seattle, Spokane, Tacoma, and other covered cities process ADUs through standard residential permits.

Permitting & plan check

Major cities like Seattle, Tacoma, and Spokane run their own permit portals with published review timelines; Seattle's SDCI process is known for thorough multi-cycle review. Smaller jurisdictions often contract plan review to third-party firms, which can speed or slow timelines depending on backlog. Pre-application conferences are common for complex commercial projects.

Good to know

• Western Washington is in Seismic Design Category D due to the Cascadia subduction zone.
• Washington enforces the Washington State Energy Code, one of the most stringent commercial energy codes in the country.
• Seattle enforces its own Seattle Energy Code that exceeds state requirements.

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