5 county-level rules, plus city-specific rules for 8 cities in King County, Washington.
Verified from official government sources
In unincorporated King County, one Accessory Dwelling Unit is permitted per primary detached dwelling or townhouse under KCC 21A.08.030.B.7.a. ADUs are limited to 1,000 sq ft of heated floor area (plus up to 1,000 sq ft unheated). Either the ADU or the primary unit must be owner-occupied, and a Notice on Title must be recorded before permitting.
King County Code (KCC) Title 21A, 21A.08.030 B.7.a. — Residential Accessory Dwelling Units (ADUs) Zoning Standards (March 2026)
ADUs located within the Urban Growth area are allowed to have up to two ADUs per property. These can be attached to the primary dwelling unit, detached from the primary dwelling unit, or any combination. ADUs located outside of the Urban Growth area (rural area zones) are limited to one ADU per property. ADUs located in the Rural Area zones (RA-2.5, RA-5, RA-10, and RA-20) are only allowed a de...
In unincorporated King County, a one-story detached storage shed of 200 square feet or less, accessory to a residential or agricultural use, is exempt from a building permit under the Washington State Residential Code (WSRC R105.2, WAC 51-51). Sheds must still meet K.C.C. Title 21A zoning setbacks, lot coverage, and critical-area rules.
Converting a garage to living space in unincorporated King County needs a building permit and counts as an attached ADU under WA HB 1337. Must meet energy, egress, smoke-alarm, and ceiling standards.
Carports in unincorporated King County require a building permit and must meet zoning setbacks. Detached carports under 200 sqft may be exempt from permit but still must meet setbacks and critical-area rules.
Unincorporated King County allows tiny homes on foundations that meet the Washington State Building Code (RCW 19.27 / 2021 IRC Appendix Q). Tiny houses on wheels are treated as RVs and limited under KCC Title 21A zoning.
8 cities in King County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
9 verified rules • ADU Impact Fees, ADU Owner Occupancy
9 verified rules • ADU Impact Fees, ADU Owner Occupancy
9 verified rules • ADU Impact Fees, ADU Owner Occupancy
5 verified rules • ADU Rules, Carport Rules
5 verified rules • ADU Rules, Carport Rules
5 verified rules • ADU Rules, Carport Rules
5 verified rules • ADU Rules, Carport Rules
5 verified rules • ADU Rules, Carport Rules
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