ADU Rules in Bellingham, Washington

Bellingham (population ~95,000) is the county seat of Whatcom County and the largest city in far northwestern Washington State. Located on Bellingham Bay just 21 miles south of the Canadian border, Bellingham is home to Western Washington University (WWU), a nationally recognized state university with approximately 16,000 students. The city serves as a gateway to the San Juan Islands (Anacortes ferry terminal is 30 miles south), the North Cascades, and BC, Canada. Bellingham is known for its progressive community values, outdoor recreation culture, and a growing economy. The city is subject to Washington's statewide ADU reform laws.

Washington State ADU Law: HB 1337 (2023)

Washington HB 1337 (2023) is the foundational statewide ADU law that applies to Bellingham. 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 smaller than 1,000 square feet
• State law limits how restrictive local setback, lot coverage, and height standards for ADUs can be

Washington HB 1110 (2023) accompanies HB 1337, requiring Bellingham to allow duplexes and middle housing in residential zones — consistent with the city's generally progressive planning approach.

Bellingham is subject to the Washington Growth Management Act (GMA) and has a comprehensive plan with an Urban Growth Area coordinated with Whatcom County.

Bellingham Local ADU Code

Bellingham's Planning and Community Development Department administers local zoning and building permits. Local ADU regulations must comply with HB 1337 and cannot be more restrictive than state law allows.

Permitted ADU Types

• Detached ADU: A separate structure in the rear or side yard
• Attached ADU: An addition to the primary dwelling sharing at least one wall
• Internal/Converted ADU: Conversion of existing garage, basement, or attic space

Up to 2 ADUs per lot are permitted (one attached/internal + one detached).

Size Limits

State law prohibits Bellingham from requiring ADUs smaller than 1,000 square feet. Confirm current maximum size limits with Bellingham's Planning Division for your specific zone.

Setbacks

HB 1337 limits Bellingham's ability to impose setbacks that would prevent reasonable ADU construction. Setback requirements vary by zoning district. Verify current standards with the Permit Center.

No Owner-Occupancy Requirement

State law prohibits owner-occupancy mandates. Bellingham cannot require property owners to live on-site to rent an ADU.

Critical Areas Ordinance (CAO)

Bellingham's CAO is extensive, protecting:

• Bellingham Bay and shoreline: Regulated by both the CAO and the Shoreline Master Program (SMP). Shoreline jurisdiction extends 200 feet inland from Bellingham Bay and other marine shorelines
• Salmon-bearing streams: Several streams flow through Bellingham to the bay, with fish habitat buffers
• Wetlands: Bellingham has significant wetland areas in lower elevations
• Steep slopes: Surrounding hillsides have landslide hazard designations
• Aquifer recharge areas: The Nooksack River basin provides important aquifer recharge

Check Bellingham's GIS mapping system to identify CAO-regulated features and shoreline jurisdiction before designing an ADU.

Seismic Hazard

Bellingham and Whatcom County face earthquake risk from the Cascadia Subduction Zone (CSZ), which runs offshore and could produce a magnitude 9.0+ earthquake. Coastal and low-lying areas near Bellingham Bay have soft marine sediments susceptible to liquefaction and tsunami inundation from a Cascadia event. Washington DNR publishes tsunami inundation maps for Washington's coast. All ADU construction must meet Washington State Building Code seismic requirements. Sites near the bay or on soft soils should have geotechnical review.

Proximity to Canada

Bellingham's position 21 miles from the US-Canada border (Peace Arch and Pacific Highway crossings) has no direct effect on local building or zoning regulations. However, Bellingham serves as a major staging area for border-adjacent commerce, and the cross-border economy contributes to local housing demand.

Permit Process

1. Check your property's zoning, shoreline jurisdiction, and CAO constraints using Bellingham's GIS portal
2. Schedule a pre-application meeting with Bellingham's Planning Division for constrained sites
3. Prepare construction plans meeting Washington State Building Code requirements
4. Submit permit application at cob.org/permits or in person at City Hall, 210 Lottie Street
5. Plan review and permit issuance
6. Construction inspections
7. Final inspection and Certificate of Occupancy

Contact Bellingham Planning: (360) 778-8300 | cob.org/planning

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