King County, WA Accessory Structures: Tiny Homes (2026)

Tiny homes in unincorporated King County are regulated under Title 16 (Building and Construction) and Title 21A (Zoning) of the King County Code, combined with the Washington State Building Code (RCW 19.27) and the 2021 International Residential Code including Appendix Q for Tiny Houses. A permanent tiny home built on a foundation of 400 square feet or less qualifies under IRC Appendix Q, which relaxes ceiling height, loft, and stair requirements. The home must still meet state energy code, plumbing code, and electrical code. In most residential zones (R-1 through R-8), a tiny home may serve as a primary dwelling if it meets minimum dwelling unit standards, or as an accessory dwelling unit (ADU) under King County ADU rules allowing one attached and one detached ADU per single-family lot. Washington HB 1337 (2023) requires GMA-planning jurisdictions to allow at least 2 ADUs per lot by right, which King County has implemented. Tiny houses on wheels (THOWs) are classified as recreational vehicles under Washington law and may not be used as permanent dwellings on residential lots unless placed in a licensed RV park or manufactured/mobile home park. Temporary occupancy of a THOW during active construction of a primary residence is sometimes permitted with a temporary use permit. Critical areas including wetlands, streams, steep slopes, and salmon habitat (KCC 21A.24) impose additional siting restrictions. Septic systems must be approved by Seattle-King County Public Health under KCC Title 13.