Building an ADU, whether a backyard cottage, a unit over the garage or a suite inside the house, is easier in Washington than it has ever been. A 2023 state law, HB 1337, now requires cities to allow two ADUs on most residential lots and removed many of the old barriers. Each city still writes its own details, though, and they differ more than most homeowners expect.
This guide covers the state rules and how Seattle, Bellevue, Kirkland, Redmond, Mercer Island, Sammamish and Issaquah apply them, as of October 2026. ADU rules are still being updated in several cities, so confirm the details for your lot with the city before you design.
What state law requires
For cities inside the urban growth area, which covers Seattle and the Eastside cities in this guide, RCW 36.70A.681 requires that cities:
- Allow two ADUs per lot wherever a single-family home is allowed: one attached and one detached, two attached, or two detached.
- Not require the owner to live on the property. Cities can still limit short-term rentals.
- Allow at least 1,000 square feet. A city cannot cap ADUs below that size.
- Allow at least 24 feet of height for an ADU, unless the main house has a lower limit.
- Treat ADUs like the main house for setbacks, lot coverage, tree retention and design review, and allow detached ADUs right on a lot line that borders an alley.
- Allow conversions of existing structures, such as a detached garage, even if they do not meet current setbacks.
- Limit parking requirements: no parking can be required within half a mile walking distance of a major transit stop, and elsewhere no more than one space per unit on lots under 6,000 square feet and two on larger lots.
- Charge no more than half the impact fees of the main house, and not require street improvements.
Lots with critical areas such as wetlands or steep slopes can be treated differently, and cities can restrict ADUs on lots without sewer.
Seattle
- Two ADUs per lot, attached or detached. Seattle’s newer zoning also allows several homes per lot in its Neighborhood Residential zones, and ADUs count toward that total.
- Size: at least 1,000 square feet is allowed, with more room for larger units under the 2026 code.
- Height: up to 32 feet in most Neighborhood Residential zones, including detached ADUs in the rear yard.
- Parking: none required for ADUs.
- Owner occupancy: not required.
- Permits: SDCI offers pre-approved backyard cottage plans, which can shorten review. See the SDCI ADU page.
Bellevue
Bellevue updated its ADU code (LUC 20.20.120) in 2025. Some older city web pages still say detached ADUs are not allowed. That is no longer true.
- Up to two ADUs per lot, attached or detached.
- Size: up to 1,200 square feet, with some flexibility when an ADU takes a full floor of the house or converts an existing accessory building. Garage and unheated storage area, up to a limit, does not count.
- Height for detached ADUs: 24 feet, or 28 feet when built over an existing accessory structure.
- Parking: none for ADUs under 1,000 square feet or within half a mile of a major transit stop; otherwise one space per ADU.
- Short-term rentals: the city does not allow ADUs to be used as short-term rentals.
- Permits: ADUs need building permits, applied for through MyBuildingPermit.com.
Kirkland
- Two ADUs per home, in any combination of attached and detached (KZC 115.07).
- Size: up to 1,200 square feet.
- Height: the same as a single-family home in the zone.
- Parking: none near major transit; elsewhere one space may be required when more than one ADU is built, with waivers in some cases.
- Ownership: ADUs can be owned separately, for example through a unit lot subdivision.
- Waterfront lots on Lake Washington have extra limits, such as one ADU in some shoreline areas and detached ADUs placed on the landward side of the house. See the Kirkland ADU toolkit.
Redmond
- Two ADUs or tiny homes per residential lot (RZC 21.04.1110), attached, inside the house or detached.
- Size: up to 1,000 square feet for an ADU, with exceptions for a full floor of the house and some larger lots; tiny homes up to 400 square feet.
- Alleys: detached ADUs can sit on a lot line that borders an alley or right-of-way.
- Older Redmond ADU handouts predate the state law. Height and parking follow the zone and state limits, so confirm them with the city for your lot.
Mercer Island
- Up to two ADUs per lot, attached or detached, under interim rules adopted in 2025. Permanent rules are expected to follow.
- Size: 220 to 1,000 square feet, not counting garage area.
- Design: new detached ADUs and additions match the roof pitch, siding and windows of the house.
- Parking: none within half a mile walk of the light rail station; otherwise one space per unit on lots up to 6,000 square feet and two on larger lots.
- Owner occupancy: no longer required. ADUs can be sold separately as a condominium or on a unit lot.
Sammamish
- Two ADUs per lot with a single-family home: attached, detached or a converted structure such as a garage (SDC 21.05.010).
- Size: up to 1,000 square feet, or up to half of the house when the ADU is inside it.
- Parking: no extra space when the property already has four or more.
- Separate entrance: a 2025 city interpretation requires each ADU to have its own entrance.
Issaquah
- Up to two ADUs, attached or detached, with a single-family home or middle housing (IMC 18.500.090).
- Size: up to 1,000 square feet, and the main home must be at least 25% larger than the largest ADU.
- Height: the same as the main home.
- Parking: the city’s submittal checklist calls for an additional off-street space and a walkway to the ADU entrance.
- Faster path: Issaquah offers a self-certification program for detached ADUs designed by a Washington-licensed architect.
Quick comparison
| City | ADUs per lot | Max size | Parking |
|---|---|---|---|
| Seattle | 2 | 1,000 sq ft or more | None required |
| Bellevue | 2 | 1,200 sq ft | None under 1,000 sq ft or near transit, otherwise 1 |
| Kirkland | 2 | 1,200 sq ft | None near transit; 1 in some cases |
| Redmond | 2 | 1,000 sq ft | State limits apply |
| Mercer Island | 2 | 1,000 sq ft | None near light rail, otherwise 1–2 |
| Sammamish | 2 | 1,000 sq ft | None if 4+ spaces exist |
| Issaquah | 2 | 1,000 sq ft | 1 additional space |
Attached, detached or a conversion?
- Inside the house, such as a basement or a full floor, is usually the most cost-effective, because the structure and roof already exist.
- A garage conversion reuses an existing building and is allowed even if the garage does not meet today’s setbacks.
- A new detached ADU gives the most privacy and design freedom, and it is the biggest project: foundation, utilities, a full building and often a new sewer and water connection.
What to plan for
- Utilities: sewer, water and electrical capacity often decide where an ADU can go and what it costs.
- Trees and slopes: large trees, steep slopes and wetlands can limit placement, especially in Medina, Mercer Island and Kirkland.
- Fees: permit fees, impact fees (capped at half of a new home’s) and utility connection charges.
- Design: a well-designed ADU should feel like part of the property, not an afterthought. We design ADUs in 3D so you can see the layout, light and views before anything is built.
How we build ADUs
NW Expert Builders handles ADUs from start to finish: a site visit to check zoning, utilities and placement, design and drawings, permits, construction and final inspection. Learn more on our ADU and DADU page, see room additions, or read our guide to remodel permits on the Eastside.
Frequently asked questions
Can I build two ADUs on my lot?
In most of Seattle and the Eastside, yes. State law requires cities to allow two ADUs per lot where a single-family home is allowed, with exceptions for some lots with critical areas or no sewer.
Do I have to live on the property?
No. Cities can no longer require the owner to live in the main house or the ADU, although they can limit short-term rentals.
How big can an ADU be?
At least 1,000 square feet everywhere covered by the state law. Bellevue and Kirkland allow up to 1,200 square feet.
Do I need to add parking?
Often not. No parking can be required near major transit, and Seattle requires none for ADUs. Elsewhere, cities can require at most one or two spaces depending on lot size.
Can I sell an ADU separately?
In some cities, yes, through a condominium or a unit lot subdivision. Kirkland and Mercer Island allow it; Seattle does not allow ADUs to be split off by subdivision.
Where do I start?
Call (206) 825-4153 or schedule a free consultation. We will look at your lot and tell you what you can build.
