Accessory Structures in King County, WA (2026)

3 county-level rules, plus city-specific rules for 6 cities in King County, Washington.

Verified from official government sources

Note: These county rules apply to unincorporated areas of King County. If you live within an incorporated city, that city's own rules apply: see the city list below.

ADU Rules

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, WA ADU Rules (Unincorporated)

Some Restrictions

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...

Full ordinance details →Official source ↗

ADU Impact Fees

Unincorporated King County exempts accessory dwelling units from school impact fees under County Code § 21A.43.070.A.9, so building an ADU triggers no per-unit school fee that a new single-family home would owe.

King County ADU School Impact Fee Exemption

Few Restrictions

King County Code § 21A.43.070.A.9

A. The following are excluded from the application of the impact fees: ... 9. Accessory dwelling units as defined in K.C.C. 21A.06.350 and K.C.C. 21A.08.030.B.7.a.

Full ordinance details →Official source ↗

Shed & Carport Rules

In unincorporated King County, one-story tool sheds and playhouses under 200 square feet skip a building permit if the eave stays 3 feet from other buildings and the shed sits at least 5 feet away. Bigger sheds need a DPER building permit.

King County Shed Size, Setback, Permit Rules

Some Restrictions

King County Code § 16.02.240

A building permit shall not be required for the following: Building: 1. One-story detached one- and two-family residential accessory buildings used as tool and storage sheds, playhouses... not including garages or other buildings used for vehicular storage, if: 1.1 The floor area does not exceed 200 square feet (18.6 m2); 1.2 The roof eave does not project closer than three feet to any portion ...

Full ordinance details →Official source ↗