How Big Can an ADU Be in Washington State: Size, Height, Coverage

Under Washington State law, an accessory dwelling unit must be allowed to reach at least 1,000 gross square feet, and cities and counties can permit larger. So the honest answer to how big an ADU can be in Washington State is: at minimum 1,000 square feet anywhere covered by the Growth Management Act, and often more depending on your lot’s zoning, coverage limits, and whether the unit is attached to your house or freestanding.

The 1,000-Square-Foot Floor

House Bill 1337, signed in 2023, set a statewide size floor for ADUs. Local governments planning under the Growth Management Act cannot cap an ADU below 1,000 gross square feet of floor area.¹ Before the change, some cities held ADUs to 600 or 800 square feet, which made a real one-bedroom hard to design. That is no longer allowed.

Cities and counties are bringing their codes into compliance on a rolling schedule tied to their next periodic comprehensive plan update, with many jurisdictions facing a compliance deadline of December 31, 2026.² Until your local code is updated, the state standards effectively override any tighter local number. If your city still has an 800-square-foot cap on the books, you are entitled to the state floor.

Two ADUs on One Lot

The other piece of the size question is how many units you can add. Inside urban growth areas, cities and counties must allow two ADUs on any lot where a single-family home is permitted. You can build one attached and one detached, two attached, or two detached, and two detached units may share a single structure or sit in separate buildings.¹

Each of those units can go up to the locally allowed maximum. On a lot with room for it, that adds up to 2,000 or more square feet of new living space beyond your existing house. On lots of 2,000 square feet or less, a jurisdiction is allowed to cap the total at two ADUs alongside the principal home, and any lot still has to meet the minimum size for a principal unit before any ADU is allowed.

Height: Building Up Instead of Out

State law also bars local governments from setting an ADU roof height limit below 24 feet, unless the principal home itself faces a lower cap, in which case the ADU matches the house.¹ Twenty-four feet is enough for a two-story unit, and on a small lot that is often how homeowners fit a full-sized ADU without running into coverage limits below.

ADUs cannot be held to stricter standards than the primary home. Setbacks, yard coverage, tree retention, entry door placement, and design review must all be equal to or more lenient than what applies to the main house.¹ If your house is allowed to sit five feet off the property line, so is your ADU. Detached ADUs can be built right at the lot line when the property fronts a public alley, unless the city plows that alley for snow.

Existing structures like detached garages can be converted to ADUs even if they violate current setback or lot coverage rules. An older garage that has stood for years does not become non-conforming the moment you finish it as living space.

Local Rules That Shrink Your Maximum

The 1,000-square-foot floor keeps cities from boxing you into a tiny unit. Your ceiling, though, is set by whatever local dimensional standard binds first on your particular lot. Several usually interact.

Floor Area Ratio

Some cities use a floor area ratio, or FAR, to control overall density. FAR is the total floor area of all structures on the lot divided by the lot area. If your lot is 5,000 square feet and the FAR limit is 0.5, total building area across everything on the property maxes out at 2,500 square feet. An 1,800-square-foot house then leaves only 700 square feet for an ADU under that cap. Under state law, FAR and similar dimensional standards must apply equally to ADUs and principal units, so a city cannot single ADUs out for tighter treatment.²

Lot Coverage

Lot coverage limits the percentage of your land occupied by all structures combined. A city might set the ceiling at 40 or 50 percent. Even if an ADU fits inside the size and FAR limits on its own, its footprint has to fit within the combined coverage cap along with the house and garage. On a tight urban lot where existing buildings already fill most of the allowed coverage, going vertical under the 24-foot height allowance is usually the way to reach a full-sized unit.

Attached Versus Detached

Whether the ADU is built into the main house or stands separately can change the allowed size. Some cities set different caps by type. Spokane currently allows up to 975 square feet for a detached ADU or 75 percent of the primary building’s size, whichever is greater, but only 800 square feet for an attached interior ADU. Tacoma caps ADU habitable area at 1,000 square feet regardless of configuration. Local rules vary, and reading your specific code is the only reliable way to find your number.

Percentage of the Primary Home

Some ordinances tie the ADU maximum to a percentage of the primary home’s floor area, often 40 to 50 percent. On a large house this rarely binds. On a modest 1,200-square-foot home with a 40 percent cap, the ADU would come out to 480 square feet, which is below the state floor. The state’s 1,000-square-foot minimum should override that local cap once the jurisdiction updates its regulations, and homeowners caught in the gap may need to push back against outdated code language.

Where Some Washington Cities Stand

City rules are moving quickly as jurisdictions rewrite codes to match state law. A snapshot of where a few larger cities sit right now:

Tacoma: maximum habitable area of 1,000 square feet, excluding garage and non-living areas.
Spokane: attached ADUs capped at 800 square feet; detached ADUs allowed up to 975 square feet or 75 percent of the primary building, whichever is greater.
Seattle: ADUs are restricted in size and count toward density limits, with regulations being updated to align with state law.

Spokane’s 800-square-foot cap on attached ADUs falls below the state floor, so it will need to change as the city brings its code into compliance. That lag exists across the state. Verify whether your city has adopted the updated standards before you assume any particular number from the municipal code is final.

Parking, Which Can Eat Your Yard

Parking mandates matter for size because required parking spaces compete with the ADU for the same yard. State law limits how much parking a city can demand. Within half a mile of a major transit stop, no off-street parking can be required for an ADU at all.² Elsewhere, lots under 6,000 square feet cannot be required to provide more than one space per ADU; lots of 6,000 square feet or more are capped at two spaces per ADU.

An earlier statute, RCW 36.70A.698, already barred cities from requiring off-street parking for ADUs within a quarter mile of a major transit stop; HB 1337 stretched that radius to half a mile.³ If your property sits inside either zone, a parking pad will not compete with your unit for buildable space. Cities also cannot require public street improvements as a condition of ADU permitting.

Confirming the Number for Your Lot

Start with the planning, community development, or permitting page on your city or county website and search the municipal code for “ADU” or “accessory dwelling unit.” Most jurisdictions publish maximum sizes, setback tables, and lot coverage limits online. Read those alongside your lot’s actual dimensions.

If your city has not yet updated its ADU regulations, the state standards in RCW 36.70A.680 and 36.70A.681 give you a floor you can rely on: 1,000 square feet, 24 feet of height, and two units per lot in urban areas.⁴ The Washington Department of Commerce keeps a plain-language summary of the state requirements.⁵ Call the local planning desk when the code is ambiguous. Staff can walk through lot-specific constraints that will not be obvious from the text alone.

1
Washington State Legislature. Washington Code RCW 36.70A.681 – Accessory Dwelling Units Limitations
2
MRSC. Accessory Dwelling Units (ADUs)
3
Washington State Legislature. Washington Code RCW 36.70A.698 – Accessory Dwelling Units Off-Street Parking
4
Washington State Legislature. Washington Code RCW 36.70A.680 – Accessory Dwelling Units Local Regulation
5
Washington State Department of Commerce. Accessory Dwelling Units (ADUs)