Accessory Structures in Bellingham, WA

If you live in Bellingham or are thinking about moving there, accessory structures are one of those things you probably won't think about until they affect you directly. Bellingham has 7 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

ADU Impact Fees

Bellingham waives its multimodal transportation impact fee for ADUs built inside an existing single-family home or accessory building, if the space predates May 21, 2018.

Key details: Exempt fee: Multimodal transportation impact fee. Cutoff date: Area built by May 21, 2018. Excluded: Garage conversions, added floor area. Same exemption: Also applies to park impact fee.

No exemption for garage conversions or new floor area: the multimodal TIF and park impact fee are assessed on that added scope at the rate in effect when the building permit is filed.

Bellingham is more permissive than most cities when it comes to adu impact fees. That said, there are still limits.

Garage Conversions

Bellingham lets homeowners convert an existing detached garage into an accessory dwelling unit even if the garage itself violates today's setback or lot-coverage standards, under BMC 20.10.036(B)(15). The converted unit still needs an ADU permit, a building permit, and full compliance with the city's health and life-safety codes.

Key details: Governing section: BMC 20.10.036(B)(15). Setback/coverage relief: Existing violations excused for conversion. Max ADU size: 1,000 sq ft (B)(6)(a). Max ADUs per lot: Two, any configuration (B)(5)(a). Health/safety compliance: Must meet BMC Titles 15 & 17 (B)(13).

Converting a garage without an ADU permit and building permit is a zoning code violation subject to the penalty and enforcement provisions of Chapter 20.52 BMC, which can include civil penalties and a compliance order from the planning and community development department. A conversion that fails the health and life-safety standards of BMC Titles 15 and 17 can be red-tagged by the building official under those titles' construction codes until brought into compliance.

ADU Rental Restrictions

Bellingham allows short-term rentals in attached ADUs citywide but bars them in detached ADUs on residential-single lots, permitting detached-ADU rentals only in commercial, multi-family and urban village areas.

Key details: Attached ADU rentals: allowed citywide. Detached ADU rentals: barred in residential single. Permits per site: one, primary unit or ADU. Review trigger: reconsidered at 4% vacancy.

Operating a short-term rental out of a detached ADU in a residential single area, or holding permits for both the primary unit and its ADU on one site, violates BMC 20.10.037(B)(5) and (B)(5)(d), enforceable under Chapter 20.52 BMC.

This is not one of those rules that cities tend to ignore. Bellingham actively enforces its adu rental restrictions requirements.

Shed & Carport Rules

Bellingham caps detached accessory buildings like sheds and garages at 800 square feet and one story (12 feet), and lets them sit in rear yards or the rear 22 feet of a side yard.

Key details: Max size: 800 sq ft (larger needs conditional use permit). Max height: One story, 12 feet. Placement: Rear yard or rear 22 ft of side yard. Governing code: BMC 20.30.100 (Residential Single zone).

Building or expanding an accessory structure beyond 800 square feet or 12 feet in height without a conditional use permit, or placing it outside the allowed rear/side yard area, is a code violation requiring after-the-fact permitting or removal.

ADU Owner Occupancy

Bellingham does not require owner-occupancy to build or rent an ADU -- state law (HB 1337) bars that -- but an owner-occupied attached ADU is exempt from the rental registration fee under BMC 6.15.040(B)(4).

Key details: State law: HB 1337 bars ADU occupancy mandates. City ADU rule: No owner-occupancy required (20.10.036). Fee exemption: Owner-occupied attached ADU only. Max ADUs per lot: Two, attached and/or detached.

There's no owner-occupancy violation for ADU permitting itself; failing to register the rental property at all, or to pay the fee when not exempt, triggers the penalties under BMC 6.15.150 -- $200-$500 per day.

The rules around adu owner occupancy in Bellingham lean permissive, but that does not mean anything goes.

ADU Rules

Bellingham allows up to two accessory dwelling units per lot, capping each ADU at 1,000 square feet and detached units at 24 feet tall under Bellingham Municipal Code § 20.10.036.

Key details: ADUs per lot: Up to two, attached or detached. Max ADU size: 1,000 sq ft per unit. D-ADU + ancillary cap: 1,800 sq ft (2,000 for two). D-ADU setback: 5 ft side/rear; none on alley. D-ADU height: 24 ft (or 12 ft, alt. definition).

Building a third ADU on a lot, exceeding the 1,000-square-foot per-unit cap, or a D-ADU exceeding 1,800 (or 2,000) square feet without a conditional use permit violates BMC 20.10.036 and can block permit approval or occupancy.

Tiny Homes

Bellingham's building code locally adopts IRC Appendix Q, the tiny-house construction standard, giving builders a legal path to permit compliant tiny homes as accessory structures.

Key details: Adopting section: BMC 17.10.020, § 118, item 2. Standard adopted: 2021 IRC Appendix Q. State baseline: IRC required; Appendix Q optional. Minimum fine: $100 civil infraction.

Building a tiny house that doesn't meet Appendix Q, or building without a permit, is subject to civil infraction fines up to $2,050 per violation ($100 minimum) under Construction Administrative Code § 114.4.

The Bottom Line

Compared to many U.S. cities, Bellingham gives residents more room on accessory structures. 2 of the 7 rules here are rated permissive. But permissive does not mean unregulated. There are still requirements, and the city does enforce them when violations are reported.

These rules come from Bellingham's publicly available municipal code. For complete penalty schedules, exemption details, and answers to common questions, see the individual ordinance pages throughout this guide.