ADU Rules in Bellevue, Washington

Bellevue (population ~155,000) is the largest city on Seattle's Eastside and the second-largest city in King County. Located on a plateau between Lake Washington to the west and Lake Sammamish to the east, Bellevue is one of the most expensive real estate markets in the United States. The city is a major tech hub — home to numerous technology company campuses and close to Microsoft's world headquarters in adjacent Redmond — and faces extreme housing cost pressures. ADUs offer a critical pathway to adding housing on existing residential lots.

Washington State ADU Law: HB 1337 (2023)

Washington HB 1337 (2023) is the foundational statewide ADU law that applies to Bellevue. Its key requirements:

• Cities must allow up to 2 ADUs per lot — one attached/internal + one detached — by right on any lot with a single-family home
• No owner-occupancy requirement permitted
• No ADU-specific impact fees allowed
• Cities may not require ADUs to be smaller than 1,000 square feet
• Setback, lot coverage, and height restrictions for ADUs are limited by state law

Washington HB 1110 (2023) accompanies HB 1337, requiring Bellevue to allow duplexes and middle housing in residential zones — a significant change for an affluent suburb historically dominated by large single-family homes.

Bellevue is subject to the Washington Growth Management Act (GMA), requiring a comprehensive plan, Urban Growth Area planning, and zoning consistent with regional growth targets.

Bellevue Local ADU Code

Bellevue's Development Services Department administers local zoning and building permits. Local ADU regulations operate within the framework established by HB 1337.

Permitted ADU Types

• Detached ADU: A standalone structure in the rear or side yard, separate from the primary home
• Attached ADU: An addition to the primary dwelling sharing at least one wall
• Internal/Converted ADU: Space converted within the existing home — garage, basement, or upper floor

Lots may have up to 2 ADUs (one attached/internal + one detached) simultaneously.

Size Limits

State law prohibits Bellevue from requiring ADUs smaller than 1,000 square feet. Consult Bellevue Development Services for current maximum size limits in your specific zoning district.

Setbacks

HB 1337 limits Bellevue's ability to impose setbacks that would effectively block ADU construction. Specific setback requirements vary by zoning district. Verify current standards with Development Services before finalizing ADU design.

No Owner-Occupancy Requirement

Washington state law prohibits owner-occupancy mandates. Bellevue cannot require property owners to live on-site to rent an ADU to tenants.

Critical Areas Ordinance (CAO)

Bellevue's Critical Areas Ordinance is comprehensive and significantly affects ADU siting. The city has numerous streams draining to Lake Washington and Lake Sammamish — many with salmon-bearing status — as well as wetlands, steep slopes, landslide hazard areas, and seismic hazard zones. CAO buffers from streams and wetlands can be 50–200 feet and effectively limit where on a lot an ADU can be built. Bellevue's GIS mapping system identifies CAO-regulated features. Always check CAO constraints early in the design process.

Seismic Hazard

Bellevue and the entire Puget Sound region face seismic risk from the Cascadia Subduction Zone (CSZ), which could produce a magnitude 9.0+ earthquake. Bellevue has areas with fill soils, soft sediments, and steep slopes susceptible to landslides during strong shaking. USGS and Washington DNR seismic hazard maps identify higher-risk zones. ADU foundations must meet Washington State Building Code seismic requirements. Sites on steep slopes, near stream corridors, or on fill soils should have geotechnical evaluation.

Permit Process

1. Check your property's zoning and CAO constraints using Bellevue's online GIS system (maps.bellevuewa.gov)
2. Schedule a pre-application meeting with Bellevue Development Services for complex sites
3. Prepare construction plans meeting Washington State Building Code requirements
4. Submit permit application online at permits.bellevuewa.gov or in person at City Hall
5. Plan review and permit issuance
6. Required inspections during construction
7. Final inspection and Certificate of Occupancy

Contact Bellevue Development Services: (425) 452-6800 | bellevuewa.gov/city-government/departments/development-services

Disclaimer: This guide provides general information based on Washington HB 1337 (2023) and Bellevue's municipal code as of April 2026. Zoning regulations change frequently. Always verify current requirements with Bellevue Development Services before making development decisions. This is not legal advice.