ADU rules in King County, WA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
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's unincorporated ADU regulations are codified at KCC 21A.08.030.B.7.a (Residential Uses — Accessory Dwelling Unit). One ADU is allowed per primary detached dwelling or townhouse unit in nearly every zone except Mineral (M) and Industrial (I). Properties within the Urban Growth Area may have up to two ADUs per lot under the 2024 state and county updates implementing WA HB 1337. ADUs may not exceed 1,000 sq ft of heated floor area (basements and attics meeting certain conditions are excluded). Either the ADU or the primary dwelling must be owner-occupied; the owner-occupancy requirement is enforced through an Affidavit and Notice on Title filed with King County Records and Licensing Services before the building permit is issued. Setbacks, height, and lot coverage follow the underlying zone, with detached ADUs typically capped at 25 feet in height in residential zones. Septic system capacity must be verified through Public Health — Seattle & King County for parcels not on sewer. Design review is administrative — no public hearing for code-compliant ADUs.
Constructing an ADU without a permit, or operating an unpermitted second unit, violates KCC 21A and the building code. Code Enforcement may issue a Notice and Order requiring removal, after-the-fact permitting, or restoration. Daily civil penalties accrue under KCC 23.20 until corrected, and a violation lien may be recorded against the property.
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See how King County's adu rules rules stack up against other locations.
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