What are the tiny house laws in Washington?

Your Washington tiny house must meet the State Building Code, even on wheels¹. Washington defines a tiny house as a dwelling no larger than 400 square feet with a kitchen, bathroom and sleeping/living area¹. You still need local approval for the foundation, installation and location¹.

Wheels alone do not determine which Washington rules apply to your home¹. A wheeled structure outside Washington’s tiny-house definition may be an RV or park model¹. Before ordering a unit, ask your local building department how it classifies the design and what placement rules apply¹.

L&I approves construction of your Washington tiny house if you build it away from its final site¹. If you build where you will live, your local building department handles approval¹. For off-site construction, L&I issues its insignia only after approving plans and inspecting construction¹. Arrange inspections before you cover any work¹.

Email FAS1@lni.wa.gov for instructions on submitting plans electronically and paying¹. Ask L&I what you will pay for plan review, inspections and the insignia for your design. Ask your local building department separately about foundation, installation and placement fees¹. Before buying a completed or used tiny house, ask that department whether it will accept the building¹.

Washington’s 400-square-foot limit sets a maximum; smaller designs do not automatically qualify¹. Your tiny house must have the facilities and meet the construction standards in Washington’s definition¹. Ask your local building department which residential code rules apply to your proposed floor plan.

No single minimum total floor area is confirmed here for every Washington tiny-house design. Before you pay for plans, ask the building official about room dimensions, sleeping lofts, stairs and emergency escape. The State Building Code applies across Washington’s counties and cities, including residential construction in unincorporated areas⁴.

Washington’s ADU protections apply within designated urban growth areas, with exceptions set by state law². Qualifying lots in zones allowing single-family homes must allow at least 2 ADUs in the configurations specified by state law². Those protections do not give you the same right on rural land outside an urban growth area².

Your Washington ADU must still meet health, building, environmental and on-site sewage requirements². Critical areas, buffers and sewage limits can restrict where you put an ADU². Before designing a backyard tiny house, ask your planning office whether your parcel qualifies. Ask whether your proposed unit will be approved as an ADU or as the parcel’s primary dwelling.

Your Washington tiny house needs a lawful sewage arrangement as well as building approval^1,3. You need a local health officer’s permit before installing, modifying, expanding or connecting to an on-site sewage system, except minor repairs³. Your septic application includes the drinking-water source, soil and site evaluation, site plan and system design³.

Local health rules can require new Washington development to connect to public sewer under certain conditions⁵. Ask your local health office whether your proposed home can use existing septic capacity or needs a new design. Before committing to a completed house, confirm that the local building department will accept that exact unit¹. Get local placement approval separately from L&I’s off-site construction approval¹.