Washington ADU Law (HB 1337): Two ADUs Per Lot Explained

Washington's ADU law, HB 1337 (2023), requires cities and counties planning under the Growth Management Act to allow two accessory dwelling units on every lot in their urban growth areas that allows a single-family home, in any mix of attached and detached. They must allow ADUs of at least 1,000 ft², may not require the owner to live on site, and are limited in how much parking they can demand. Together with HB 1110 (middle housing), it is one of the most permissive state housing packages in the country, and a 2025 parking law, SB 5184, goes further still. Law current as of October 2026.

This post sets out what HB 1337 requires, how it fits with HB 1110, when the rules apply in your city, and an 800 ft² two-bedroom ADU plan that sits well inside the size floor. For other states, see ADU laws by state: Hawaii's Act 39 is also moving to two ADUs per lot, and Montana's SB 528 caps one ADU at 1,000 sq ft or 75% of the house.

What HB 1337 requires

HB 1337 was signed in May 2023 and is codified in RCW 36.70A.680 and 36.70A.681. The main requirements on cities and counties, as summarized by the state Department of Commerce and by cities implementing it:

The law does not override critical areas, shoreline rules, or health requirements for septic systems, and cities can still apply ordinary building, fire and stormwater rules.

SB 5184: less parking still

Parking is where the rules are still moving. SB 5184 (chapter 204, Laws of 2025) bars cities of more than 30,000 people, and counties, from requiring any off-street parking for homes under 1,200 sq ft, which covers most ADUs, and from requiring more than one space for a single-family house. Cities and counties of 50,000 or more must put it in place within 18 months of July 27, 2025, so by late January 2027; those of 30,000 to 50,000 have three years, to mid-2028. Until your city adopts it, the HB 1337 parking limits in the table apply.

When it applies in your city

HB 1337 first gave cities and counties until six months after their next periodic comprehensive plan update. For King, Pierce, Snohomish and Kitsap counties and the cities in them, that update was due at the end of 2024, so their ADU rules had to be in place by June 30, 2025. A 2025 law, SB 5558, now has the ADU rules take effect with the update itself for everyone else: December 31, 2025 for the Clark, Thurston, Whatcom and Skagit group (now passed), December 31, 2026 for the Spokane, Yakima, Benton and Chelan group, and June 30, 2027 for the remaining counties. If a city misses its deadline, the state requirements apply directly. Your city's planning page will say whether its HB 1337 ordinance has been adopted.

Seattle was ahead of the law: since 2019 it has allowed two ADUs on most single-family lots without an owner-occupancy requirement, and its newer rules implement the state laws. Across the border, Vancouver's laneway house rules take a similar approach to backyard cottages.

How HB 1110 (middle housing) fits in

HB 1110, passed in the same session, requires larger cities to allow "middle housing" (duplexes up to sixplexes, townhouses, cottage housing and similar) on lots zoned for houses:

• Cities of 75,000 or more: at least 4 units per lot, 6 within a quarter mile of a major transit stop or where two units are affordable.
• Cities of 25,000 to 75,000: at least 2 units per lot, 4 near major transit or with one affordable unit.
• Smaller cities inside the contiguous urban growth area of the largest city in a county of more than 275,000 people: at least 2 units per lot.

ADUs may count toward these unit totals under local codes. The practical effect is that on a typical urban Washington lot you can now choose between a house plus two ADUs, a duplex or more, or a cottage cluster. If you are weighing a duplex instead, see our Oregon middle housing guide: the two states wrote similar laws.

An 800 ft² two-bedroom ADU

FloorGen's 800 sq ft 2-bed ADU template is a detached unit measuring 40 × 20 ft to the outside of the walls (800.1 ft² gross), with 2×6 exterior walls. Its net floor area inside the walls is 700.5 ft². Two bedrooms are at one end, a bath and laundry alcove in the middle, and an open kitchen, dining and living room at the other.

At 800 ft² gross it sits well under the 1,000 ft² floor, so no Washington city that complies with HB 1337 can refuse it on size. That leaves room to grow: lengthen it by 5 ft to about 1,000 ft² and you can add a third bedroom or a larger living room. On a narrow city lot, a 20 ft wide unit leaves space for the required side setbacks, and on an alley lot HB 1337 lets the ADU sit at the alley line.

Two ADUs on one lot: layout tips

• Attached plus detached is the common pair: a basement or above-garage unit in the house and a backyard cottage. The attached unit needs fire separation from the main dwelling under the residential code.
• Two detached cottages work on deeper lots. Keep the doors facing a shared path rather than each other, and plan where the bins and bikes go.
• Stormwater and utilities often decide what fits: ask early whether each ADU needs its own sewer connection or meter.
• Stairs in a two-storey ADU or one above a garage take more floor space than people expect; see the stair code guide for the rise, run and headroom numbers.

The building code side

Washington adopts the International Residential Code statewide with state amendments (WAC 51-51); the 2021 IRC edition is in force with amendments, and a move to the 2024 IRC is proposed for 2027. A detached ADU is built like a small house: habitable rooms of at least 70 ft² and 7 ft wide (2021 IRC R304), an emergency escape opening in each sleeping room (R310), smoke and CO alarms, and the Washington State Energy Code, which is stricter than many states. See legal bedroom requirements for the escape-opening sizes.

FloorGen's IRC profile checks room areas and widths, ceiling heights, bedroom escape windows, stairs and door swings as you draw. It does not check zoning, setbacks, the energy code or alarms, so treat it as a layout tool and take the plan to your city's permit counter.