Washington HB 1337, in plain English
What the 2023 statewide ADU law actually requires of cities — and what it means if you own a single-family lot in the Puget Sound.
What the law does
Engrossed Substitute House Bill 1337, signed into law in 2023, requires Washington cities planning under the Growth Management Act to allow at least two accessory dwelling units on any lot zoned for a single-family residence — one attached and one detached, or two of either type. Cities cannot impose owner-occupancy requirements, cannot require off-street parking within a half-mile of major transit, and must allow ADUs of at least 1,000 square feet.
It also caps impact fees at 50% of those charged for the principal unit and prohibits design standards stricter than those applied to the primary house.
Sources:Washington State LegislatureMunicipal Research and Services Center of Washington
Why it matters in Seattle and the Eastside
Seattle had already preempted most of HB 1337's substantive rules in 2019. The bigger impact lands in Bellevue, Kirkland, Redmond, and Tacoma, which had to bring their codes into compliance by mid-2025. Owners in those cities can now legally build a DADU plus an AADU on the same lot, where previously only one ADU was permitted.
MRSC tracks adoption status across jurisdictions and is the cleanest place to confirm where a given city stands.
Sources:Municipal Research and Services Center of Washington
What HB 1337 does not do
It does not override critical-areas regulations, shoreline rules, or floodplain requirements. It does not allow ADUs on lots that fail underlying lot-size minimums in zones where minimums are still legal under the GMA. And it does not by itself unlock condo-style fee-simple sale of the ADU — that requires separate unit-lot subdivision authority, which Seattle and a few other cities offer but most do not.
Practical takeaway
If you own a single-family lot anywhere in a GMA-planning city, you almost certainly have an ADU pathway in 2025 that did not exist in 2022. The remaining variability is local: setbacks, height, lot coverage, tree retention, and stormwater. Those are the items a feasibility study should still verify before design begins.
