ADU regulations in King County operate on three layers: Washington State law (HB 1337 / RCW 36.70A.680-681) sets the baseline, Seattle's municipal code (SMC 23.44.041 and 23.44.025) governs properties within city limits, and King County code (Title 21A) covers unincorporated areas.
The guides below break down each layer in plain English so you know exactly what applies to your property. Not sure where to start? Get a free feasibility study — we'll analyze your zoning, setbacks, and critical areas in 48 hours.
HB 1337 Explained
How Washington's landmark ADU law changed everything — two ADUs per lot, no owner occupancy, minimum size floors, and more.
Read guideSeattle Municipal Code (SMC 23.44.041)
Plain-English breakdown of Seattle's ADU ordinance — what's allowed, size limits, setbacks, and permit types through DCI.
Read guideKing County Code (Title 21A)
Unincorporated King County ADU regulations — lot sizes, zoning districts, UGA rules, and 2025 updates through DPER.
Read guideSeattle vs King County
Side-by-side comparison of ADU regulations inside Seattle city limits vs. unincorporated King County.
Read guideShoreline Management Act
How SMA affects ADU construction near Lake Washington, Lake Union, Green Lake, and Puget Sound shorelines.
Read guideOwner Occupancy Rules
HB 1337 eliminated owner-occupancy mandates statewide. What this means for Seattle ADU owners and investors.
Read guideTwo ADUs Per Lot
Yes — you can build two ADUs in Seattle. Rules for attached, detached, and mixed configurations.
Read guideShort-Term Rental (Airbnb) Rules
Seattle STR license requirements, ADU-specific rules, and King County regulations for short-term ADU rentals.
Read guideRelated Resources
Our team handles ADU permitting from start to finish. See our permitting cost breakdown for transparent pricing.
Important Disclaimers
- 1. Shoreline Management Act areas add extra review for lots within 200 ft of Lake Washington, Lake Union, Green Lake, or Puget Sound. Always check.
- 2. HOA CC&Rs can add restrictions beyond city/county rules.
- 3. Critical areas (wetlands, streams, steep slopes, flood zones) can limit ADU eligibility on otherwise qualifying lots.
- 4. Laws are actively evolving. All content includes last-updated dates and is reviewed quarterly.
- 5. This site is not legal advice. Always verify current regulations with the appropriate permitting department (Seattle DCI or King County DPER).
Last updated: March 2026.
