ADU rules in King County

In the urban area (inside the urban growth area), up to two ADUs per lot are allowed in any configuration, including one or two detached structures. In the rural area or on natural resource lands, a detached ADU is allowed only when there is no more than one primary dwelling on the lot and the lot is at least 3,200 sq ft (rural towns) or meets the minimum lot area for the zone (elsewhere); otherwise the ADU must be in the same building as the primary dwelling. ADUs are prohibited entirely in the Forest (F) zone.

Attached ADUs must share at least one common wall with the primary dwelling and appear contained within one structure; a breezeway or covered pathway connection only counts as attached if it is under 10 feet, shares a common wall with both units, is completely enclosed, and is heated. Units wholly within a basement or attic of the primary dwelling are also allowed (and exempt from the size cap).

ADUs not wholly contained within an existing dwelling also may not exceed the base height for the zone (21A.08.030.B.7.a.(2)).

The current ADU standards in KCC 21A.08.030.B.7.a (as amended through Ord. 19881, 2024) contain no owner-occupancy requirement; a full-text search of Title 21A finds none for ADUs. The county's March 2026 ADU bulletin likewise omits it. Caution: an older county permit-guide FAQ page still says one unit must be owner-occupied; that reflects pre-2024 code.

Applies countywide (urban and rural). Urban-area ADUs are also exempt from public street improvements (B.7.a.(7)(c)).

ADUs are a residential accessory use marked P (permitted, condition 7) across residential and rural zones in the use table, subject to the B.7.a standards rather than a hearing; applications go through a standard building permit via MyBuildingPermit.com. Before permit approval the applicant must record a notice on title identifying the unit as accessory (B.7.a.(5)).

Title 21A contains no ADU rental-term or short-term-rental provisions. County business-licensing titles outside the zoning code were not audited for general short-term-rental rules.