The City of Bellingham governs ADU construction through BMC 20.10.036 — the Accessory Dwelling Unit section of its municipal code. On December 8, 2025, the city adopted an ordinance update to align with Washington's HB 1337 mandates, making it easier than ever to build ADUs in Bellingham.
This guide covers the key rules in plain English. For the state-level context, see our HB 1337 explainer.
Where ADUs Are Allowed
ADUs are permitted in most areas within Bellingham city limits — specifically in zones where they're listed as a permitted use. This includes all Residential Single (RS) zones, which cover the majority of Bellingham neighborhoods. For the full permit process, see our Bellingham ADU permit guide.
Lake Whatcom Watershed Exception
ADUs are NOT allowed in the Lake Whatcom Watershed (Chapter 16.80 BMC). This affects Silver Beach, Sudden Valley, and other lakeside neighborhoods. Learn more.
Source: BMC 20.10.036(B)(1)
How Many ADUs Can You Build?
2
ADUs per lot
Any
Configuration
1
Primary dwelling max
You can build up to two ADUs per lot in conjunction with the primary dwelling. Any configuration is allowed: 2 attached, 2 detached, or 1 of each. ADUs are exempt from density limitations.
The lot may not contain more than one primary dwelling unit.
Source: BMC 20.10.036(B)(3)(a)
Size Limits
General Maximum: 1,000 sq ft
The standard maximum ADU size is 1,000 square feet. For detached ADUs (D-ADU), this floor area includes all attached ancillary space like garages, workshops, or sheds.
Maximum height is 24 feet.
Basement Conversion Exception
For basement attached-ADU (A-ADU) conversions: the 1,000 sq ft size limit does not apply if the primary dwelling has been on the site for 5+ years. This makes older homes particularly good candidates for basement ADUs.
Oversized D-ADU (Conditional Use)
A detached ADU may exceed 1,000 sq ft only with a Conditional Use Permit (CUP) approved by the Hearing Examiner. This involves a public hearing and 500-foot mailing list notice.
Source: BMC 20.10.036(B)(6)
Setback Requirements
ADUs must comply with all standard zoning code provisions for the primary dwelling — including height, setbacks, floor area, and open space requirements.
Reduced Setback Option
A detached ADU may be built less than 5 feet from a property line if:
- A joint agreement with the adjoining property owner is recorded, OR
- The Planning Director determines impacts are negligible (based on slope, existing development, etc.)
Minor Modifications
Applicants may request minor modifications to development/design standards. This is not a variance — it does not need to meet variance criteria. The Director may grant it if the site is physically constrained, the modification won't harm neighborhood compatibility, and won't be detrimental to public welfare.
Source: BMC 20.10.036(B), BMC 20.10.036(B)(2)
Don't want to navigate the permit process yourself? Our ADU permitting service handles everything — zoning verification, plan review, and building permits — for a $1,500 deposit that's applied to your project cost. See our full cost breakdown for Whatcom County.
Permit Types
Type I — Administrative Review
The fastest path. Can be submitted with your building permit. No neighbor notification required. This is the standard process for most ADU projects.
Best for: Most ADU projects (1-2 units on a residential lot)
Type II — Enhanced Review
Required for projects with 7 or more total dwelling units (including ADUs) under the Interim Middle Housing Ordinance 2025-12-032. Requires a 500-foot mailing list notice.
Best for: Large multi-unit projects (rare for standard ADU builds)
Conditional Use Permit (CUP)
Required for oversized detached ADUs exceeding 1,000 sq ft. Involves a public hearing before the Hearing Examiner, plus 500-foot mailing list and neighborhood representative notification.
Best for: Large detached ADUs over 1,000 sq ft
Source: BMC 21.10.040, BMC 20.10.036
Owner Occupancy
Currently, owner occupancy is required in residential single zones — the owner must submit an affidavit acknowledging this requirement. A temporary suspension is available for illness, death, or unforeseeable events (1 year, renewable once).
This is changing: HB 1337 eliminates owner-occupancy mandates statewide effective July 1, 2025. This requirement is expected to be fully removed upon Bellingham's complete HB 1337 compliance. See our owner occupancy guide for details.
Source: BMC 20.10.036(B)(4)(b)
Other Key Rules
ADU Access
Main entrance must have direct access to a street via a lighted pedestrian path, driveway, or alley.
Source: BMC 20.10.036(B)(3)(b)
Subdivision
ADU land cannot be subdivided from the primary dwelling land. However, the ADU may be segregated in ownership (sold as a condo).
Source: BMC 20.10.036(B)(4)(a)
Existing Structure Conversion
Allowed — you can convert existing garages, basements, or other structures into ADUs, even if they don't meet current setback or lot coverage standards. See our garage conversion and basement ADU services.
Source: December 2025 ordinance update
Related Guides
Official Sources
- Bellingham Municipal Code ADU section: bellingham.municipal.codes
- City of Bellingham ADU page: cob.org
- Bellingham Planning Dept: (360) 778-8300, [email protected]
Last updated: March 2026. Not legal advice. Verify current regulations with Bellingham Planning Department.
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