Can I add a backyard cottage in Seattle, Bellevue, Kirkland or Redmond?

What House Bill 1337 changed for King County cities

For years each city wrote its own ADU rules. Seattle allowed backyard cottages on many lots. Bellevue allowed attached units, like a basement apartment, and required the owner to live on the property. A buyer with plans for a rental cottage had to learn a new code every time the search crossed the lake.

The Legislature narrowed those differences in 2023. House Bill 1337 applies to every city in King County, and it requires each one to allow two accessory dwelling units on lots zoned for a single house, as long as the lot meets the minimum size for the main house. The two units can be attached, detached, or one of each. Cities can no longer require the owner to live on site. They can’t require off-street parking for an ADU close to a major transit stop. And a city can no longer block an accessory unit from being sold on its own as a condominium, though setting that up is a legal project and a question for a real estate attorney.

The law also protects existing buildings. A detached garage that sits closer to the property line than today’s setbacks allow can generally still be converted, even if it breaks today’s setback or lot coverage rules. Cities had to bring their codes in line as part of their most recent comprehensive plan updates, which is why several Eastside codes were rewritten recently. What the state left to each city (size limits, height, setbacks, design standards, fees) is what decides most individual lots.

Seattle: two accessory units on most residential lots

Seattle moved first. Its 2019 changes allowed two accessory units on most lots zoned for houses, in what the city now calls Neighborhood Residential zones, and dropped both the owner-occupancy rule and the parking requirement. The city has kept adjusting these rules as it adopts the newer state housing laws, so the combinations and size limits that apply to a specific lot today come from SDCI, the Seattle Department of Construction and Inspections.

In Seattle a detached unit is usually called a DADU or backyard cottage. An attached one, such as a basement apartment or an addition, is an AADU. Two things make Seattle easier than people expect. SDCI offers a set of pre-approved DADU plans that can shorten design and review. And many neighborhoods, from Ballard and Wallingford to parts of West Seattle and Beacon Hill, were platted with alleys, which give a cottage its own front door and a much simpler route for construction and utilities.

The hills and the trees work the other way. Much of the city sits on slopes, and some of those slopes are mapped as environmentally critical areas. Seattle’s tree protection rules, updated in 2023, also reach many of the large trees that grow in old backyards. A big Douglas fir or a mature maple in the middle of the lot can decide where a cottage goes, or whether it goes at all.

Bellevue, Kirkland and Redmond: detached cottages are now on the table

For Eastside buyers the change is larger. Bellevue, which for a long time allowed only attached units, has recently updated its code to permit detached ones. Kirkland has allowed detached ADUs for years and adjusted its rules to match the state law. Redmond updated its code under the same law.

Eastside lots are often bigger than Seattle’s. A 1960s rambler in Lake Hills or a house on a deep lot in Bridle Trails may have room behind it that a Ballard lot never will. What slows Eastside projects down is different. Fewer streets have alleys, so the cottage and its utilities have to come past the main house. Each city has its own tree retention rules. Newer subdivisions often carry recorded covenants and an HOA. And on Mercer Island, in Somerset and on Kirkland’s Finn Hill, slopes and critical areas come into play.

Issaquah, Sammamish, Mercer Island, Renton and Kent sit under the same state law with codes of their own. If you’re comparing a house in Redmond with one in Sammamish, expect two different answers on how big and how tall a cottage can be. The communities page covers how those places differ on everything else, from school districts to the drive to work.

Reading a listing for ADU potential

“ADU potential” in listing remarks is a hope, and sometimes it’s already in the price. Before it changes what you’d pay, check the lot itself and King County’s parcel map for these:

• Alley. Alley access makes construction, a separate entrance, parking and utility runs simpler. Without one, everything passes the main house, and access requirements for emergency responders can limit where a cottage sits.
• Slope. A flat backyard is cheaper to build on. A lot that falls away, or sits in a mapped steep slope or landslide-prone area, may need a geotechnical report before anything else.
• Trees. Note the big ones and where they stand. Their root zones count, and removing one may take city approval.
• Side sewer. A new unit usually ties into the existing sewer line or needs a new one. An old line that is cracked or undersized becomes part of the budget, and a sewer scope during inspection will show its condition.
• Power and water. A second unit can mean a larger panel or a new service from Seattle City Light or Puget Sound Energy, plus a conversation with the water provider, which on the Eastside may be the city or a local water district.
• Lot shape and what’s already there. A narrow or oddly shaped lot, the footprint of the main house, a big deck and an existing garage all use up lot coverage and setbacks.
• Covenants. Recorded covenants and HOA rules are a separate layer from zoning. Read them in the title report, and ask an attorney if they seem to conflict with state law.

The lots with the best views are often the hardest ones to add a cottage to. I’ve written separately about buying a view home on a steep lot, including how slope rules limit decks, additions and ADUs.

Buying a house that already has an ADU

A finished cottage out back is a different kind of purchase. Start with the permit history. Seattle’s permit records are searchable through SDCI, and Bellevue, Kirkland and Redmond each keep their own. You’re looking for a permit issued for an accessory dwelling unit and a final inspection that closed it out. A building permitted as a garage or a studio and later fitted with a kitchen is something else, and it deserves a careful conversation with the city before you commit.

Then look at how the unit is used. If someone lives there, ask for the lease along with the rent and deposit records, and talk with a real estate attorney about what you take on at closing. Seattle requires rental housing to be registered with the city, so ask whether the unit is. If you plan to count rent toward qualifying for your loan, ask your lender early how it treats ADU income, because lenders differ. Tell your insurance agent there is a second unit and whether it’s rented, so the policy fits the property.

Appraisal can be the tricky part. There may be few nearby sales of houses with permitted ADUs, which leaves the appraiser little to compare against. And if the idea of selling a unit separately interests you, the ownership questions overlap with how Seattle’s newer townhomes are held, which I cover in unit lot versus condominium townhomes.

Confirm with the city before you pay for the potential

Every city in the county has a permit or planning desk that will talk through ADU questions for a specific address, and many offer some form of pre-application meeting. Use them during your inspection period, or before you write the offer if the cottage is the reason you want the house.

Bring the parcel number, a few photos of the yard, and the questions that matter to your plan: how big the unit can be, how tall, where on the lot, what the utilities will need and what fees apply. If the answer is close, pay an architect or builder who works in that city for a quick feasibility look.

Then decide what the potential is worth to you. A lot that clearly works is worth more to a buyer who wants a cottage. A lot that might work is a bet, and it should be priced like one.