House Bill 1337, signed into law on April 6, 2023 and effective July 5, 2023, fundamentally changed how Washington cities and counties must treat Accessory Dwelling Units. The law establishes statewide minimum standards that local jurisdictions cannot restrict below — effectively creating a "right to build" for ADUs across the state.
Key dates: The statewide compliance deadline for cities and counties was June 30, 2025. Seattle had already adopted most of these reforms in 2019 via CB 119544 (Ordinance 125791). If a jurisdiction doesn't comply, state standards under RCW 36.70A.535 automatically take effect.
What HB 1337 Requires
Two ADUs Per Lot
All lots in urban growth areas that allow single-family homes must permit at least two ADUs. Any configuration is allowed: 2 attached, 2 detached, or 1 of each. ADUs are exempt from density limitations.
What this means: A Seattle homeowner can now build a backyard cottage AND convert their basement — both as permitted ADUs on a single lot. See our guide to two ADUs per lot for configuration options.
Source: RCW 36.70A.680
Owner Occupancy Eliminated
Cities and counties may NOT require the owner to live in the ADU or the primary residence. No more annual affidavits. Homeowners can rent both the main home and ADU(s) independently. Effective July 1, 2025 statewide.
What this means: Investors can purchase a property, build an ADU, and rent both units — without ever living on-site. Read our owner occupancy guide for the full timeline.
Source: RCW 36.70A.681(1)(b)
1,000 Sq Ft Minimum Floor
Local governments cannot set maximum ADU size below 1,000 sq ft. This is a floor — cities can allow larger, but cannot restrict below this threshold.
What this means: Previously, some jurisdictions capped ADUs at 800 sq ft or a percentage of the main home. That's no longer allowed.
Source: RCW 36.70A.681(1)(c)
Impact Fee Limits
Impact fees for ADU construction cannot exceed 50% of the fees for the principal unit.
What this means: Lower upfront costs make ADU projects more financially accessible. See our full ADU cost breakdown for current Seattle and King County numbers.
Source: RCW 36.70A.681(1)(a)
Parking Relaxation
No parking required if the ADU is within 1/2 mile of a major transit stop. This applies across Seattle near Link Light Rail (1 Line) stations and RapidRide lines (A/B/C/D/E).
What this means: Eliminating a parking space saves $5,000-$15,000 in construction costs and frees up buildable area on your lot.
Source: RCW 36.70A.681(1)(e)
Existing Structure Conversion
Jurisdictions must allow conversion of existing legal nonconforming structures into ADUs — even if the structure doesn't meet current setback or lot coverage standards.
What this means: That old garage that's 3 feet from the property line? You can convert it to an ADU without having to tear it down and rebuild to current setbacks. See our garage conversion services.
Source: RCW 36.70A.681(1)(d)
Condo Ownership
ADUs may be sold as condominiums independent of the principal unit. The land stays with the primary homeowner, but the ADU structure can have separate ownership.
What this means: This creates a new asset class — individual ADU units that can be bought and sold independently.
Source: RCW 36.70A.681(1)(g)
What HB 1337 Does NOT Do
Does NOT override short-term rental restrictions
Individual cities can still ban or regulate Airbnb/VRBO for ADUs. HB 1337 specifically does not prohibit local STR restrictions.
Does NOT override environmental protections
Critical areas, the Shoreline Management Act, and flood zone regulations still apply. ADUs within 200 ft of Lake Washington, Lake Union, Green Lake, or Puget Sound still face extra shoreline review.
Does NOT eliminate building code requirements
ADUs still must meet all building, plumbing, electrical, and energy code requirements. Construction permits are still required.
Does NOT override HOA restrictions (directly)
While state law generally preempts HOA restrictions that conflict with ADU law, enforcement is evolving. Some HOA communities may still attempt to restrict ADUs through CC&Rs.
Ready to take advantage of HB 1337? Our permit specialists handle the entire process — land use and building permits included. Your investment is applied as a build deposit. Or start with a full cost breakdown to understand the investment.
How Seattle & King County Implemented HB 1337
City of Seattle
Seattle was ahead of the curve — its 2019 reform (CB 119544 / Ordinance 125791) already allowed two ADUs per lot, removed owner-occupancy, and waived parking. HB 1337 locks those gains in through SMC 23.44.041 and 23.44.025, administered by DCI.
Full Seattle code guideKing County
Unincorporated King County applies HB 1337 through King County Code Title 21A, administered by DPER — two ADUs per lot in urban zones, no owner-occupancy, and the state size floor.
Full county code guideOfficial Sources
- WA RCW 36.70A.680-681: app.leg.wa.gov
- Seattle DCI ADU page: seattle.gov/sdci
- King County DPER: kingcounty.gov
Last updated: March 2026. Not legal advice. Verify current regulations with your local planning department.
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