Accessory Structures in Bellingham, WA: What Residents Actually Need to Know

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 5 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

Shed Rules

BMC 20.30.100 governs accessory buildings (including sheds) in Residential Single (RS) areas: max one story or 12 ft height, max 800 sq ft area (larger only by conditional use permit), and located in the rear yard or the rear 22 ft of an interior side yard. The 2018 Washington State Building Code (RCW 19.27) — based on the 2018 IBC/IRC adopted by WAC 51-50 and 51-51 — exempts one-story detached residential accessory structures up to 200 sq ft from a building permit, provided no electrical, plumbing, or mechanical systems are added. Sheds over 200 sq ft, or any shed with utilities, require a Bellingham Permit Center building permit.

Key details: Code Reference: BMC 20.30.100 + WSBC 2018. Max Height: 1 story OR 12 ft. Max Size (no CUP): 800 sq ft. Location: Rear yard or rear 22 ft of interior side yard. Habitation/Business: Prohibited (except ADUs under BMC 20.10.036).

Building without a required Washington State Building Code permit triggers a Bellingham Permit Center stop-work order, double permit fees per WAC 51-50, and potential removal order. Zoning violations (exceeding 800 sq ft without conditional use approval, violating BMC 20.30.100 location/height/use rules, using the shed for human habitation or unauthorized business) are enforced by Bellingham Code Compliance under BMC 20.30.100 with civil infractions and daily penalties.

Bellingham is more permissive than most cities when it comes to shed rules. That said, there are still limits.

Carport Rules

BMC 20.30.100 treats detached carports as accessory buildings in RS zones: max one story or 12 ft height, max 800 sq ft, located in the rear yard or rear 22 ft of an interior side yard. Attached carports follow the principal-building setbacks (front, side, rear) of the underlying RS or RM zone, with typical 5 ft side and 20-25 ft front setbacks per BMC 20.30.040. Construction requires a building permit from the Bellingham Permit Center under the 2018 Washington State Building Code (RCW 19.27) with engineered anchorage for Bellingham wind loads (~85-95 mph) and Seismic Design Category D2.

Key details: Code Reference: BMC 20.30.100 + 20.30.040 + WSBC 2018. Detached Carport Height: 1 story or 12 ft. Detached Carport Max Size: 800 sq ft (CUP required for larger). Side/Rear Setback (detached): Typically 5 ft (RS zones). Attached Carport: Principal-building setbacks (front 20-25 / side 5-7 / rear 20-25 ft).

Building without a Washington State Building Code permit triggers a Bellingham Permit Center stop-work order, double permit fees per WAC 51-50, and potential removal order. Zoning setback or location violations are enforced by Bellingham Code Compliance under BMC 20.30.100 and 20.30.040 with civil infractions and daily penalties.

If you are coming from a city with tighter rules, you will find Bellingham gives residents more flexibility on carport rules.

ADU Rules

Bellingham updated BMC 20.10.036 effective August 22, 2023 to pre-emptively adopt most provisions of WA HB 1337 (RCW 36.70A.680-696) before the state deadline. Up to TWO ADUs are permitted per single-family lot. A detached ADU (D-ADU) may be up to 1,000 sq ft of habitable floor area (1,800 sq ft total including ancillary space for one D-ADU; 2,000 sq ft for two attached D-ADUs). Height limit 24 ft. Setbacks 5 ft side/rear (zero from an alley). No off-street parking required within 0.5 mi of a Whatcom Transportation Authority major transit route. Bellingham was an early ADU adopter (first ordinance 1995).

Key details: Code Section: BMC 20.10.036 (updated Aug 22, 2023). ADUs Per Lot: 2 (attached, detached, or combination). D-ADU Habitable Floor: 1,000 sq ft max (excluding ancillary). D-ADU Total Size: 1,800 sq ft (single) / 2,000 sq ft (two attached). Height Limit: 24 ft (definition 1) / 12 ft (definition 2).

Building or occupying an ADU without a Bellingham building permit and Certificate of Occupancy is enforced by the Bellingham Permit Center under BMC Title 17 and RCW 19.27 with stop-work orders, civil penalties, and order to vacate. Zoning violations (exceeding the 1,000 sq ft habitable floor cap, exceeding 1,800/2,000 sq ft total cap, exceeding 24 ft height, violating accessory-structure setbacks) are enforced by Bellingham Code Compliance under BMC 20.10.036. Bellingham cannot enforce repealed owner-occupancy mandates or parking minimums within 0.5 mi of WTA major transit routes — RCW 36.70A.681 voids such enforcement.

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

Tiny Homes

A tiny home on a permanent foundation in Bellingham is treated either as a primary single-family dwelling (must meet BMC 20.30.040 RS zone standards) or as an Accessory Dwelling Unit under BMC 20.10.036 (1,000 sq ft habitable floor cap, 24 ft height, 5 ft side/rear setbacks). The 2018 Washington State Building Code (RCW 19.27 + WAC 51-51, effective July 1, 2021) adopts IRC Appendix Q (Tiny Houses) — allowing reduced ceiling heights, ladder access to lofts, and small-stair geometry for homes 400 sq ft or less. Tiny homes on wheels (THOWs) registered as RVs cannot serve as a permanent residence in Bellingham residential zones.

Key details: Building Code: 2018 WSBC + IRC Appendix Q (effective Jul 1, 2021). Tiny Home as Primary Dwelling: Permitted on permanent foundation if RS zone standards met. Tiny Home as ADU: Permitted under BMC 20.10.036 (1,000 sq ft habitable). HB 1337 ADU Mandate: Already implemented in BMC 20.10.036. Tiny Home on Wheels (THOW): Not permitted as residence (RV registration).

Living in a THOW or RV in a Bellingham residential zone is a zoning violation under BMC Title 20 and BMC 20.08.020, enforced by Bellingham Code Compliance with notice of violation, civil infractions, daily penalties, and order to vacate. Site-built tiny homes built without permits trigger a Bellingham Permit Center stop-work order, double permit fees per WAC 51-50, and possible removal order. Provisions inconsistent with WA HB 1337 (owner-occupancy, parking near transit) are unenforceable per RCW 36.70A.681.

Garage Conversions

Converting a Bellingham garage to habitable space requires a building permit (plus electrical, mechanical, and plumbing permits) from the Bellingham Permit Center under the 2018 Washington State Building Code (RCW 19.27). A garage-to-ADU conversion that adds a kitchen and separate entrance falls under BMC 20.10.036 ADU rules — 1,000 sq ft habitable floor max, 5 ft side/rear setbacks. Per WA HB 1337 (RCW 36.70A.681), Bellingham cannot impose an owner-occupancy mandate and cannot require off-street parking replacement within 0.5 mi of a Whatcom Transportation Authority major transit route.

Key details: Building Permit: Required (change of occupancy U→R-3). Electrical/Mechanical/Plumbing: Separate permits per WAC 51-50. Building Code: 2018 Washington State Building Code (RCW 19.27). Energy Code: 2021 WA State Energy Code (RCW 19.27A). ADU Trigger: Kitchen + separate entrance.

Building without permits triggers a Bellingham Permit Center stop-work order, double permit fees per WAC 51-50, and possible removal order. Creating an unpermitted second dwelling unit (kitchen + separate entrance) without BMC 20.10.036 ADU approval is a zoning violation under BMC 20.10.036, enforced by Bellingham Code Compliance with civil infractions, daily penalties, and stop-use orders. Provisions inconsistent with WA HB 1337 (owner-occupancy mandates, parking replacement within 0.5 mi of WTA major transit) are unenforceable per RCW 36.70A.681.

The Bottom Line

Compared to many U.S. cities, Bellingham gives residents more room on accessory structures. 3 of the 5 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.