Tiny home on wheelsNo statewide law specifically on tiny homes on wheels; wheeled units generally fall under RV-type rules in most cities. This correction: an earlier draft cited a broad 'Appendix Q adoption' as the main story here — that's not the strongest or best-documented fact about Washington. The real headline is the state's ADU mandate, below.
ADU / accessory unitHouse Bill 1337 (2023), codified at RCW 36.70A.681–.682, requires every city and county planning under the state's Growth Management Act to allow at least two ADUs per single-family lot within urban growth areas — more permissive on unit count than California or Oregon. It bars owner-occupancy requirements, bars floor-area caps below 1,000 sq ft, caps ADU impact fees at 50% of the primary unit's, and waives parking requirements within roughly a quarter-mile of major transit. Paired with HB 1110 (2023), which mandates 4–6 units per lot in larger cities, Washington is now considered one of the strongest ADU-preemption states in the country. Local codes had until mid-to-late 2024/2025 (staggered by city size) to comply.
Minimum square footageState law bars cities from capping ADUs below 1,000 sq ft within urban growth areas
Foundation requirementFoundation-built ADUs have the strongest legal footing of any state covered here; wheeled units still fall under RV-type rules in most cities