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Washington ADU law, in plain English.

Two state laws reset what Puget Sound homeowners can build: one for accessory dwelling units, one for middle housing. Here is what every city now has to allow, and what each city still decides for itself.

HB 1337 · 2023 ADUs Two ADUs per lot, size and height floors, no owner occupancy. RCW 36.70A.681 HB 1110 · 2023 Middle housing Two to six homes per lot, depending on city size and transit. RCW 36.70A.635
Residential street in Puget Sound
Family home with a large backyard deck
House beneath tall evergreens
01HOW THE RULES STACK

State law is the floor. Your city builds on top of it.

A city can be more generous than the state, never stricter. So the answer for your lot is always three layers read together: the state minimum, your city’s code, and the physical lot itself.

LAYER 03Your lot Slope, trees, alley, utilities
LAYER 02City code Setbacks, coverage, fees, process
LAYER 01 · THE FLOORState law 2 ADUs · 1,000 SF · 24 ft · no owner rule
02HB 1337 · ADUS

What every city must allow

These rules apply in cities and counties planning under the Growth Management Act, inside urban growth areas, on lots where a single-family home is allowed.

  1. 01Two ADUs per lotTwo attached, two detached, or one of each.
  2. 02At least 1,000 sq ftA city’s maximum ADU size can’t be set below 1,000 square feet.
  3. 03At least 24 ft tallDetached ADUs get a height limit of 24 feet or more, unless the main house’s own limit is lower.
  4. 04No owner occupancyThe owner doesn’t have to live in the house or the ADU.
  5. 05Parking near transitNo off-street parking can be required within half a mile of a major transit stop.
  6. 06Sale as a condoADUs can be sold separately as condominium units.
  7. 07Impact fees halvedImpact fees on an ADU are capped at 50% of the fee for the main house.
  8. 08Existing structuresA garage or other existing building can be converted even if it doesn’t meet today’s setbacks or lot coverage.
  9. 09Alley lot linesA detached ADU may sit on a lot line that borders a public alley, with limited exceptions.
  10. 10Same design rulesDesign standards can’t be stricter for an ADU than for the main house.
03LOCAL CONTROL

What your city still decides

SetbacksDistances from side, rear and front lot lines — outside the alley rule.
Lot coverageHow much of the lot buildings and hard surfaces can cover.
Short-term rentalsCities may limit or ban using an ADU as a short-term rental.
Trees & critical areasTree protection, steep slopes, wetlands and shorelines.
Permit processSubmittal requirements, review steps and permit fees.
Utility connectionsSewer, water and power rules set by the city or utility district.
04HB 1110 · MIDDLE HOUSING

More than one home per lot

Separate from ADUs, the middle-housing law lets duplexes, triplexes, townhomes and cottage clusters on lots once zoned for one house. How many depends on the city’s size.

Two-story house on a wide suburban lot
TIERCITY POPULATIONUNITS PER LOTNEAR MAJOR TRANSIT OR WITH AFFORDABLE UNITS
TIER 175,000 and up46
TIER 225,000 – 75,00024
TIER 3Under 25,000, inside a large city’s urban growth area2—
Planning a duplex or cottage cluster rather than an ADU? The trade-offs in cost, financing and resale are different. Read the guides →
05SOURCES
RCW 36.70A.681 — Accessory dwelling unitsREVISED CODE OF WASHINGTON↗ RCW 36.70A.635 — Middle housingREVISED CODE OF WASHINGTON↗ ADU and middle-housing guidanceWASHINGTON STATE DEPARTMENT OF COMMERCE↗

A plain-language summary, not legal advice. How we keep this page current →

See how the state floor plays out in your city.

Rules by city → Open the calculator