Accessory Dwelling Units (ADUs)

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ADUs are seen as a valuable component in achieving the City’s infill housing goals. There are many benefits to enabling ADU development in Bellingham including increasing housing choice and affordability as well as providing an opportunity for homeowners to earn additional income, age in place, or care for an ailing relative.

What is an ADU?

An ADU is a separate, self-contained residential unit located on the same lot as a single-family home, duplex, triplex, fourplex, townhome, or other principal unit. An ADU has all the basic facilities needed for day-to-day living independent of the principal unit, such as a kitchen, sleeping area and a bathroom. They are sometimes called “mother-in-law apartments” or “granny flats” because they are often used to house extended family. An ADU may be created as:

  • A separate structure on the lot (such as a converted garage). 
  • A separate unit within an existing home or principal unit (such as in an attic or basement). 
  • An addition to a home or principal unit (such as a separate apartment unit with a separate entrance). 

ADUs in Bellingham

The permitting process for ADUs varies depending on the scope of work as detailed in Bellingham Municipal Code (BMC) 21.10 and summarized below:

Type I

The Type I permit process is streamlined and and may include requests for minor modifications from applicable development standards. A Type I ADU application is a staff level (administrative) review and may be submitted simultaneously with a building permit application. Notification of surrounding property owners is not required. 

Type II

The Type II permit process is used per BMC 21.10.040(C)(18) and when a project consists of seven (7) or more dwelling units, including ADUs, per the Interim Middle Housing Ordinance No. 2025-12-032.

The Type II Permitting process:

  1. The applicant must submit an ADU permit application along with a mailing list of all owners of property within 500 feet. 
  2. Planning staff sends notice of the ADU application and other associated land use applications to the mailing list and applicable neighborhood representatives. 
  3. A decision is made based on the information contained in the application, the development code standards, and the written public comments received as a result of the public notice. Comments submitted should be focused on how the project proposal relates to the applicable evaluation criteria. The decision may be appealed to the Hearing Examiner. 

Type III-A

Type III-A Conditional Use Permit (CUP) process is additionally required per BMC 20.10.036(B)(6) for the following:

  • One D-ADU includes more than 800 square feet of attached ancillary space (garage, workshop, garden shed, etc.). 
  • Two attached D-ADUs include more than 800 square feet of attached ancillary space.  
  • Two attached D-ADUs with attached ancillary space will exceed 2,000 gross square feet. 

Examples needing a CUP are:  

  • One 600 square foot D-ADU with 1,100 square feet of ancillary space for a gross floor area of 1,700 square feet.  
  • One 1,000 square foot D-ADU with 950 square feet of ancillary space for a gross floor area of 1,950 square feet. 
  • Two (2) attached D-ADUs that are 400 square feet each and 1,400 square feet of attached ancillary space for a gross floor area of 2,200 square feet. 

Regardless of the above, the ADU standards limit D-ADUs to no more than 1,000 square feet each. 

The Type III-A Process: 

  1. The applicant must first hold a pre-application neighborhood meeting.   
  2. The applicant submits ADU and CUP applications along with a mailing list of all owners of property within 500 feet.   
  3. Planning staff sends notice of the applications to the mailing list and applicable neighborhood representatives. 
  4. A public hearing is held, during which the Hearing Examiner evaluates the proposal against the criteria listed in BMC 20.16.010 and any public comments received.   
  5. The Hearing Examiner then issues a decision on the proposal. The decision may be appealed to Superior Court. 

In what areas of the City are ADUs permitted?Attached and detached ADUs are allowed in most areas that allow residential development, including single-family zoned areas. ADUs are not permitted in the Lake Whatcom Watershed that drains to Basin One (BMC 16.80.040). 
How many ADUs can a property have? Eligible properties can have no more than two ADUs located on the same lot as a single-family dwelling unit, duplex, triplex, fourplex, townhome, or other principal unit. ADUs can be in any configuration of attached or detached units. 
What is the maximum size of an ADU?An ADU is limited to 1,000 square feet.
What is the height limit for ADUs?Attached ADUs must comply with the height limit of the housing type with which it is being built. Detached ADUs shall be no higher than 24 feet under BMC 20.08.020, height definition No. 1 or 12 feet under height definition No. 2
Must the owner live on site?No, owner occupancy is not required. 
May the ADUs be sold separately from the primary dwelling unit?Yes, the primary dwelling and ADU(s) may be sold separately. An example of this is condos. The City is not directly involved in the formation of a condominium, which is governed by state law (RCW 64.34 or 64.90) and administered locally at the county level.     

Note that ADUs cannot be subdivided onto lots separate from the principal unit unless the lots whereon the ADUs are located are deed restricted to provide affordable owner-occupied units per BMC 20.10.036(B)(4)(b). 
Is parking required for an ADU?No. Per Ordinance 2025-12-35 which took effect December 23, 2025, no parking is required to add ADUs to your property. Parking that is added electively is required to meet development standards per BMC 20.10.036(B)(9) and 20.12.010. 
How does a pre-existing unit become legal?Homeowners with existing non-permitted ADUs must apply to make them legal. Units that are not legalized will be subject to enforcement. Pre-existing units must meet the same requirements as new ADUs, including building code standards.   Note: Allowing more than one dwelling unit on property in single-family zones is not limited to ADUs. Ordinance 2025-12-032 applies interim development regulations to single-family and infill housing development, allowing infill housing (up to 6 units per lot) in all areas that are zoned predominantly for residential use, except on property in the Lake Whatcom Reservoir. The ordinance went into effect 12/23/2025 and will be in effect until December 4, 2026, absent further action by the City Council. Applications for ADUs and Infill Housing are available at the Permit Center. 
Employee Benefits

See the latest ADU ordinance

On December 8, 2025, the City of Bellingham amended its zoning and process codes to improve ADU standards, permitting process, and consistency with State legislation.

Watch to learn more about ADUs in Bellingham

This panel discussion provides a comprehensive overview of ADUs in Bellingham, covering key aspects of financing, permitting, construction, rental, and sale.

Resources

Local Resouces

Below are some local resources that can provide more details:

Other Resources

Contact Us

For questions or more information, please contact the Planning and Community Development Department at planning@cob.org or 360-778-8300.

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