Straight answers

Can you put a tiny house in your backyard in Washington state?

Yes in most cities, when it is permitted as an accessory dwelling unit on a foundation. RCW 36.70A.681 makes every city and county that plans under the Growth Management Act allow at least two accessory dwelling units on each lot inside an urban growth area where single-family homes are allowed, and they can be attached, detached, or one of each. The city cannot require you to live on the property, cannot set a maximum size below 1,000 square feet of gross floor area, and cannot charge impact fees above half of what the main house would pay. A tiny house on wheels is a separate question, because the state's protection for a home on wheels covers manufactured and mobile home communities, not a single backyard.

Figures verified against the sources below · October 1, 2026

01 — The longer answer

What Washington requires, and what your city still decides

What the state makes every city allow

The requirements sit in RCW 36.70A.681, added in 2023. Beyond the two-unit rule, a city cannot set a roof height limit below 24 feet unless the main house is held to less, and cannot require off-street parking within half a mile walking distance of a major transit stop. On a lot of 2,000 square feet or less it may hold the total to two accessory units. Timing is where older summaries go wrong. RCW 36.70A.680, as amended in 2025, has each jurisdiction's own rules take effect at the same time as its next periodic comprehensive plan update, and in any city or county that has not adopted them the state requirements supersede, preempt and invalidate conflicting local regulations. All of it applies inside urban growth areas only.

What a city can still say no to

The statute leaves real room. A city may prohibit accessory units on lots that are not connected to or served by public sewers, may restrict them for short-term rentals, and applies the same public health, safety, building code and environmental permitting it would apply to the main house, including rules that protect ground and surface water from on-site wastewater. Nothing in the law requires a permit where development is already restricted by critical areas or by proximity to on-site sewage systems. On a real lot the deciding questions are usually sewer, critical areas and the building permit, not whether the zoning allows a second home.

Who approves the home: Labor and Industries or your city

The state's Department of Labor and Industries describes a tiny house as a dwelling that may be built on wheels and is no larger than 400 square feet, with a kitchen, a bathroom and a sleeping and living area, built to the Washington State Building Code. Where it is built decides who approves the construction. A tiny house built in a factory, or anywhere else, and then moved to where it will be used needs a Labor and Industries insignia, which the department puts only on homes built to plans it approved and inspected during construction. A home built on the lot goes through the local building department instead. The insignia covers how the home is built. Where it may be placed is still the city's or county's decision.

A tiny house on wheels in a backyard

This is where Washington is narrower than its reputation. RCW 35.21.684 stops a city from preventing the entry, or requiring the removal, of a recreational vehicle or tiny house with wheels used as a primary residence in a manufactured or mobile home community. That protection is about communities, and it does not reach a single lot behind a house, where the city's own code decides. Spokane's code prohibits using a recreational vehicle as a permanent dwelling outside a manufactured or mobile home park, and Seattle's permitting department states that you cannot live in a tiny house on wheels on a lot within city limits. If the plan is a backyard home someone lives in year-round, a unit on a foundation permitted as an accessory dwelling unit is the route the state law protects.

Seattle, Spokane and Tacoma

All three allow two accessory units per lot. Seattle limits a unit to 1,000 square feet with up to two bedrooms and 1,200 with three or more, and requires no parking for it. Spokane allows a detached unit of up to 1,000 square feet of floor area, subject to its building footprint standards. Tacoma sets a maximum of 1,000 square feet of habitable area and requires no parking. Each figure is linked below to the city's own code, and these sections were all amended in 2025 or 2026, so read the current text before designing to it.

Outside a city: the 2026 rural rule

Land outside an urban growth area was left out of the 2023 law. RCW 36.70A.685, added in 2026, says a county may allow a detached accessory dwelling unit there, on conditions the statute lists: a gross floor area of no more than 1,296 square feet, a site within 150 feet of the main house, and the same driveway as the main house. It is permission for the county, not a right for the owner, so on rural land the county's own ordinance is the answer.

The national version of this question explains why the wheels-or-foundation split comes first in every state, and the Washington guide carries the code in force with the source for each figure.

02 — Where to go next

The Washington state guide The same question for the whole US Granny pod laws by state Building codes, state by state How certification works

03 — Sources

Each figure above links to the authority it came from.

RCW 36.70A.681(1) [2023 c 334 s 4]: a city or county may not assess impact fees on an accessory dwelling unit greater than 50 percent of those on the principal unit; may not require the owner to reside in or occupy either unit; must allow at least two accessory dwelling units on all lots in zoning districts within an urban growth area that allow single-family homes; may not set a maximum gross floor area below 1,000 square feet; may not set a roof height limit below 24 feet unless the principal unit's limit is lower. (2): no off-street parking requirement within one-half mile walking distance of a major transit stop. (3): a limit of two accessory dwelling units may apply on a lot of 2,000 square feet or less
app.leg.wa.gov
RCW 36.70A.680 [2025 c 148 s 4; 2023 c 334 s 3]: local regulations are to take effect at the same time as the jurisdiction's next periodic comprehensive plan update; where a city or county has not adopted them, the requirements supersede, preempt and invalidate conflicting local development regulations; they apply only within urban growth areas; a city may restrict short-term rental use, apply public health, safety, building code and environmental permitting, and prohibit accessory dwelling units on lots not connected to or served by public sewers
app.leg.wa.gov
RCW 36.70A.685 [2026 c 231 s 1]: counties may allow detached accessory dwelling units outside urban growth areas subject to listed conditions, including a gross floor area of no more than 1,296 square feet, siting within 150 feet of the principal unit and use of the same driveway
app.leg.wa.gov
Washington State Department of Labor and Industries, Tiny Homes: a tiny house is a dwelling that may be built on wheels and is no larger than 400 square feet, built to the Washington State Building Code; a tiny house built in a factory or elsewhere and then moved requires an L&I insignia, applied only to homes built to L&I-approved plans and inspected during construction; for placement, contact the local building department
lni.wa.gov
RCW 35.21.684(3): a city may not adopt an ordinance preventing the entry or requiring the removal of a recreational vehicle or tiny house with wheels used as a primary residence in manufactured/mobile home communities
app.leg.wa.gov
Spokane Municipal Code 17C.300.110: a detached accessory dwelling unit shall not exceed 1,000 square feet of floor area, subject to the building footprint standards in Table 17C.111.205-2. SMC 17C.300.100: two accessory dwelling units are allowed per lot. SMC 17C.319.200: a recreational vehicle is prohibited from being used as a permanent dwelling unit except in a manufactured or mobile home park
my.spokanecity.org
Seattle Municipal Code 23.42.022: no lot may have more than two accessory dwelling units; maximum 1,000 square feet for a unit with up to two bedrooms and 1,200 square feet with three or more; no parking required for an accessory dwelling unit
library.municode.com
Seattle Department of Construction and Inspections, Accessory Dwelling Units: you cannot live in a tiny house on wheels on lots in Seattle city limits
seattle.gov
Tacoma Municipal Code 13.06.080.A: two accessory dwelling units are allowed per residential lot; a maximum of 1,000 square feet of habitable area; no parking required
ecode360.com

04 — Related questions

Can you put a tiny house in your backyard in the US?Which US states let you build a granny pod as of right?What is a prefab ADU?Where can you put a tiny home in the US?Are tiny homes legal in Arizona, and where can you put one?Can you put a tiny house in your backyard in New Hampshire?

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