ADU regulations in Whatcom County operate on three layers: Washington State law (HB 1337 / RCW 36.70A.680-681) sets the baseline, Bellingham's municipal code (BMC 20.10.036) governs properties within city limits, and Whatcom County code (WCC 20.80.910) covers unincorporated areas.
The guides below break down each layer in plain English so you know exactly what applies to your property. Not sure where to start? Book a free 30-minute consultation — we'll talk through your zoning, setbacks, and critical areas.
HB 1337 Explained
How Washington's landmark ADU law changed everything — two ADUs per lot, no owner occupancy, minimum size floors, and more.
Read guideBellingham Municipal Code (BMC 20.10.036)
Plain-English breakdown of Bellingham's ADU ordinance — what's allowed, size limits, setbacks, and permit types.
Read guideWhatcom County Code (WCC 20.80.910)
Unincorporated Whatcom County ADU regulations — lot sizes, zoning districts, UGA rules, and 2025 updates.
Read guideBellingham vs Whatcom County
Side-by-side comparison of ADU regulations inside vs outside Bellingham city limits.
Read guideLake Whatcom Watershed Restrictions
Why the watershed has stricter rules and what homeowners in Silver Beach, Sudden Valley, and nearby areas need to know.
Read guideOwner Occupancy Rules
HB 1337 eliminated owner-occupancy mandates. What this means for ADU owners and investors.
Read guideTwo ADUs Per Lot
Yes — you can build two ADUs. Rules for attached, detached, and mixed configurations.
Read guideShort-Term Rental (Airbnb) Rules
STR rules for ADUs in Bellingham, Lynden, Blaine, and Whatcom County.
Read guideRelated Resources
Our team handles ADU permitting from start to finish. See our permitting cost breakdown for transparent pricing.
Important Disclaimers
- 1. Lake Whatcom Watershed is the #1 exception to nearly every ADU-friendly rule. Always check.
- 2. HOA CC&Rs can add restrictions beyond city/county rules (e.g., Sudden Valley).
- 3. Critical areas (wetlands, streams, flood zones) can limit ADU eligibility on otherwise qualifying lots.
- 4. Laws are actively evolving. All content includes last-updated dates and is reviewed quarterly.
- 5. This site is not legal advice. Always verify current regulations with the appropriate planning department.
Last updated: March 2026.
