ADU Guide · Verified 2026
Building an ADU in Washington
Washington has one of the strongest ADU laws in the country. Under HB 1337 (2023), every city and county fully planning under the Growth Management Act must allow at least two ADUs per lot in urban growth areas wherever single-family homes are allowed. Cities cannot require owner-occupancy, cannot cap an ADU below 1,000 square feet, and cannot apply setback, height, or design rules stricter than those for the main house. State law preempts any conflicting local rule.
Quick Facts
Statewide floors set by Revised Code of Washington, RCW 36.70A.680, .681 and .696 (HB 1337, 2023). Your city may be more generous but cannot go below them.
The Three Types of ADU
What you can build by right
HB 1337 sets floors that no fully-planning city or county can go below inside an urban growth area. A local government must allow at least two ADUs per lot wherever a single-family home is allowed, cannot establish a maximum floor area below 1,000 gross square feet, and cannot limit ADU height below 24 feet or below the height limit that applies to the principal home (RCW 36.70A.681). Setback requirements, yard-coverage limits, tree-retention mandates, entry-door-location rules, aesthetic requirements, and design review cannot be more restrictive for an ADU than they are for the main house. A city also cannot demand public street improvements as a condition of permitting an ADU.
How many you can have
The statute guarantees a minimum of two ADUs per lot in urban growth areas, in addition to the principal residence — so a single-family lot that meets the minimum lot size for the main home can carry up to three dwelling units total. Local rules must permit them attached, detached, or a combination, and the two units can be mixed (one attached and one detached, or two detached). The exact mix your city allows is set in its implementing code, but it cannot fall below the two-unit floor.
You do not have to live on the property
HB 1337 eliminated owner-occupancy requirements statewide — a city or county may not require the owner to live in the principal unit or in the ADU. That means you can rent out both the main house and the ADU. (Seattle had already removed its owner-occupancy rule in 2019; the state law extended that everywhere.) Short-term rental rules are still set locally, so confirm any Airbnb-style use with your city.
How approval works
ADU permits under the state framework are processed administratively — no public hearing, no discretionary design review board. If your plans meet the objective local standards (which themselves cannot be stricter than the state floor), the building department reviews for code compliance and issues the permit. Where a jurisdiction has missed its compliance deadline, the conflicting local rules are preempted and the state statute applies directly. The Department of Commerce publishes a model ADU permit checklist that cities use alongside their own.
Parking is limited near transit
A city cannot require any on-site parking for an ADU located within a half-mile of a major transit stop (RCW 36.70A.681). More broadly, Washington has curtailed parking mandates for smaller residences, and parking cannot be required in designated station areas near rail and bus-rapid-transit stops. Where none of those exemptions apply, a city may still ask for parking, but the requirement cannot exceed what it imposes on comparable housing.
Conversions get special protection
One of the most useful provisions: a city or county must allow ADUs to be converted from existing structures — including detached garages — even if those structures violate current setback or lot-coverage requirements (RCW 36.70A.681(2)(j)). That protects the many backyard garages built decades ago right up against a lot line. A detached ADU may also be sited on a lot line where that line abuts a public alley, unless the city routinely plows snow on that alley.
Selling an ADU separately
Washington law prevents a city or county from prohibiting the sale or conveyance of a condominium unit independently of the principal unit solely because it was originally built as an ADU (RCW 36.70A.681(2)(k)). That opens a path to selling an ADU as a separate condo where the property is set up for it — a step beyond what most states allow, though the practical mechanics (condominium declaration, financing) still require professional setup.
HOAs are a separate matter
State law overrides conflicting municipal zoning, but it does not automatically void private HOA covenants. Homeowners associations operate under contract law, and HB 1337 does not nullify existing CC&Rs that restrict accessory units — though new common-interest communities are barred from adopting such restrictions. If your property is in an HOA, review your CC&Rs and any architectural-review requirements before you build; some associations have updated their rules to match the new law, but many have not.
What people get wrong
Two misconceptions are common. First, that the law applies everywhere in Washington — it applies to jurisdictions fully planning under the Growth Management Act and to lots inside urban growth areas, so rural land outside a UGA is not covered by the two-ADU guarantee. Second, that the state floor is the whole story — many cities are more generous than the minimums, so your city may allow larger units or more flexibility than the statute requires. The state sets the floor, not the ceiling; check your local code for anything better.
From Idea to Move-In: The Timeline
Who Administers It
Counties With Their Own Rules
These Washington counties have verified, county-specific ADU rules that add to the state baseline:
Check Your County
Select your county for local rules, fees, and your building department's contact details:
Building It Yourself? Here's What You'll Need
Planning an ADU? Tools homeowners use in the design and layout phase:
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Check Your Local Rules
Your city may allow more than the state floor
State law sets the minimum; many Washington cities are more generous. Confirm specifics with your local planning department.
Find your county building department →Not sure if your project needs a building permit, a zoning approval, or both? See our guide to building permits vs. zoning approval.
Official Sources
ADU law changes frequently and local ordinances add detail. This guide is general information, not legal advice — always confirm with your local building department. Reviewed by the StateDataIndex Editorial Team · Updated August 2026.