Seattle vs. Bellevue: ADU rules side by side.

Seattle and Bellevue sit across Lake Washington and under the same state ADU law, so the big rules now match: two ADUs per lot, no owner-occupancy requirement, and the right to sell an ADU as a condominium. The differences are in the details, and they are the details that decide what fits on a particular lot.

The state floor both cities must meet

Washington’s 2023 ADU law (RCW 36.70A.681) requires cities inside urban growth areas to allow, on any lot where a house is allowed: two ADUs in any combination of attached and detached; a maximum size of at least 1,000 sq ft; a height limit of at least 24 feet; no owner-occupancy requirement; no off-street parking within half a mile walking distance of a major transit stop; and impact fees of no more than 50% of a new house’s. Both cities had to bring their codes in line by mid-2025.

Side by side

Bellevue figures follow the city’s 2025 middle-housing and ADU amendment as presented to its Planning Commission, adopted to meet the state’s June 30, 2025 deadline. Codes change; the linked code section governs.

What the differences mean on a real lot

Size

Bellevue’s 1,200 sq ft cap, plus the garage and storage exclusion, leaves more room for a two-bedroom unit or an ADU above a garage. In Seattle, 1,000 sq ft is enough for a comfortable two-bedroom, but the layout has to be efficient.

Parking

Neither city requires parking for a typical ADU. In Bellevue it comes into play only for larger units away from frequent transit; in Seattle it does not apply.

Experience and process

Seattle has permitted backyard cottages for years and has a pre-approved plan library; that institutional experience shows in a predictable process. Bellevue’s detached-ADU rules are new, so expect reviewers and applicants to be working out details on early projects. A complete first submittal matters even more there.

Cost

Our model puts Bellevue labor and permitting about 2% above Seattle’s. On an 800 sq ft detached ADU that is a few thousand dollars, much less than the difference a long side sewer or a sloped lot makes.

Before you design in either city

• Confirm the current code section: SMC 23.42.022 in Seattle, LUC 20.20.120 in Bellevue.
• Check for critical areas (slopes, wetlands, streams); the state ADU rules do not apply on those lots.
• Locate the sewer main and the existing side sewer before choosing where the ADU goes.

Rules for 18 cities and three counties, in the same format: ADU rules by city. The Seattle details are on our Seattle page.