King County, WA Accessory Structures: ADU Rules (2026)
ADU rules in King County, WA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Code Reference
- KCC 21A.08.030.B.7.a
- Max Heated Floor Area
- 1,000 sq ft
- Owner Occupancy
- ADU or primary unit
- Notice on Title
- Required before permit
- Septic Review
- Public Health if no sewer
Summary
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.
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 detached ADU if the property complies with the minimum lot size for the zone. Only attached ADUs may be permitted in the Agriculture (A) zone and must be accessory to a primary use of commercial agriculture on the site. An ADU is not allowed within the Forest (F) zones. ADUs are typically limited to 1000 sq ft of heated habitable floor area and 1000 sq ft of unheated floor area. No additional off-street parking spaces are required for accessory dwelling units.
Full Breakdown
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.
Violations & Fines
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.
Frequently Asked Questions
Can I build an ADU in unincorporated King County?
How big can a King County ADU be?
Do I have to live on the property to have an ADU?
Sources & Official References
Other rules in King County
How King County compares: Easiest Cities to Build an ADU·Compare King County to another location·View the Washington accessory structures overview
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