Can I build a DADU under Washington HB 1337 in Seattle?

Washington's Engrossed Substitute House Bill 1337 (2023) preempted the bulk of Seattle's restrictive accessory dwelling unit regime when it took effect in 2024, opening detached accessory dwelling units (DADUs) on virtually every single-family parcel inside the city. Before HB 1337, Seattle attached lot-coverage caps, owner-occupancy requirements, and off-street parking mandates that quietly killed dozens of viable DADU projects each year. The state law overrides those layers and forces cities of 25,000-plus to permit at least two ADUs (one attached, one detached) on any lot otherwise zoned for a single-family home.

For Seattle homeowners, the practical effect is that a typical 5,000 sq ft Capitol Hill, Wallingford, Greenwood, or West Seattle lot can now host a 1,000 sq ft detached cottage even when the existing house already maxes the previous 35-percent lot coverage limit. RCW 36.70A.681, the ADU section of the Growth Management Act as amended by HB 1337, also caps impact fees at 50 percent of what a primary unit would pay and prohibits SDCI from requiring an additional off-street parking stall within a half mile of frequent transit, which covers most of central and north Seattle.

Budget-wise, expect $180,000 to $400,000 all-in for a finished one-bedroom DADU in 2026 dollars. The wide range reflects whether the build is stick-framed on slab, modular, or a fully custom architect-designed unit; site conditions on Seattle's hilly, often Critical-Areas-encumbered lots can add $30,000 to $80,000 of geotechnical and shoring cost. Permit timelines under the streamlined HB 1337 review now average 4 to 10 months from intake to issuance, down from the 9-to-14-month slog that was typical pre-2024.

The one regulatory trap that still trips owners up: HB 1337 does not exempt DADUs from Seattle's Critical Areas Ordinance (SMC Title 25.09) or from Landmarks review inside historic overlays. A Magnolia steep-slope lot or a Harvard-Belmont contributing structure will still trigger the longer review path, regardless of the state preemption.