Washington ADU and Middle Housing Changes Explained for Homeowners

For decades, most residential lots in Washington's cities allowed one house and, at most, one small accessory unit under tight conditions. Two laws passed in 2023 changed the starting point. Many cities now have to allow more homes on lots that used to allow one.

That gives many homeowners new options, such as a rental unit, space for family, or developing or selling part of the property. Whether any of it fits your lot depends on your city's current code and on the site itself.

• ADU (accessory dwelling unit): a smaller, complete home on the same lot as a main house.
• AADU (attached ADU): inside or attached to the main house, such as a basement or above-garage unit.
• DADU (detached ADU): a separate building, often a backyard cottage.
• Middle housing: principal homes at a scale between a single house and an apartment building, including duplexes through sixplexes, townhouses, stacked flats, courtyard apartments and cottage housing.

An ADU is accessory to a main house. Middle housing is more principal homes on a lot. The two are regulated by different parts of state law.

HB 1337 (now RCW 36.70A.680–.681) applies to cities and counties planning under the Growth Management Act, inside urban growth areas, in zones that allow single-family homes. In those places, local rules must:

• allow at least two ADUs per lot: one attached and one detached, two attached, or two detached;
• not cap ADU size below 1,000 square feet, or ADU height below 24 feet (unless the main house's limit is lower);
• not require the owner to live on the property;
• not require off-street parking within half a mile walking distance of a major transit stop, with limited exceptions;
• not charge ADU impact fees above 50% of those for the main house;
• not prohibit selling an ADU separately as a condominium unit.

These rules do not apply to lots in designated critical areas or their buffers, and certain watersheds are also excluded. Size, height and parking limits set the floor for city rules. They do not mean a particular backyard can fit two units.

HB 1110 (now RCW 36.70A.635, amended in 2024 and 2025) sets minimum densities for lots zoned predominantly residential. The baseline depends on city size:

Cities in the first two groups must allow at least six of the nine middle housing types. They can choose to allow more than the state minimum. Critical areas and some other lots are excluded, and a city may apply the density to at least 75% of its single-family lots instead of all of them. The numbers above are state minimums, not the rule in any particular city.

The bills passed in 2023. Each city's obligation starts with its own periodic comprehensive plan update, so cities adopted new code at different times, and some have changed it since. After a jurisdiction's deadline passes, the state rules override conflicting local rules.

For any property, what matters is what state law requires, what the city has adopted, and what is in effect today. A city web page or news story describes one moment; the current code is what applies.

The two laws do not stack automatically. Under RCW 36.70A.635, a city may count ADUs toward its middle housing density, and it does not have to allow ADUs or middle housing beyond that required density. A lot with a 4-unit baseline does not automatically also get two ADUs on top. How your city combines them is in its own code.

Even where a use is allowed, the project still has to be buildable. Lot size, access, utilities, slopes, trees, drainage, fire access and building code all still apply.

A new home can be owned separately from the main house in a few ways, and each works differently:

• Condominium ownership: the lot stays whole and the units are owned separately. HB 1337 stops cities from prohibiting this for ADUs.
• Unit lot subdivision: a city process that creates separate unit lots within a development, where the city's code provides for it.
• Lot division: creating a new, separate legal lot. A 2025 state law on residential lot splitting is being phased in; Commerce guidance says cities must adopt it by July 27, 2027, unless their periodic update is due in 2027.

None of these is guaranteed for a given backyard. Which one, if any, is available depends on the city and the site.