Building an ADU in Bellingham

What you can build in Bellingham

Bellingham has allowed ADUs since 1995. An ADU can sit on a lot with a single-family home, duplex, triplex, fourplex, townhome or other main home. You can have up to two, attached (A-ADU) or detached (D-ADU). Examples include a converted garage, a unit in an attic or basement, or an addition with its own entrance.

ADUs are allowed in "All areas that allow residential development (except for the Lake Whatcom watershed )," including residential single, residential multi and urban village zones. The detailed rules are in Bellingham Municipal Code 20.10.036. The city's ADU page and ADU Quick Guide, both updated September 17, 2026, summarize them. [1][2]

Size, height and setbacks

Size. Each ADU can be up to 1,000 square feet. One detached ADU may also have up to 800 square feet of attached ancillary space, such as a garage, workshop or shed, if it has an outside door. Going past that needs a Type III-A conditional use permit, and even then each detached ADU stays capped at 1,000 square feet.

Height. An attached ADU can be 35 feet under height definition #1 or 20 feet under definition #2. A detached ADU can be 24 feet under definition #1 or 12 feet under definition #2. The city explains the two definitions in its building height guide.

Setbacks.

• Attached ADU: front 10 feet, rear 10 feet (0 if on an alley), interior side 5 feet (0 if on an alley), corner-lot flanking side 10 feet
• Detached ADU: front 10 feet, rear 0 feet, interior side 5 feet (0 if on an alley), corner-lot flanking side 10 feet. A detached ADU can sit in the rear 32 feet of the lot with no side setback in that section.

The code itself words the detached ADU setbacks differently. BMC 20.10.036 says "A five-foot side and rear yard setback shall be provided," with no setback needed from an alley, and that a detached ADU "may be located in a rear yard and in the rear 22 feet of an interior side yard." The Quick Guide shows a 0-foot rear setback and the rear 32 feet. Confirm the setbacks for your lot with the Permit Center before you design.

At least 30 percent of the lot must be open space. [1][2][3][4]

Parking and owner occupancy

No parking is required for an ADU. If you choose to add parking, it must meet the standards in BMC 20.10.036(B)(9) and 20.12.010.

You don't have to live on the property. Owners used to have to record a "Covenant for Owner Occupancy." If one is still recorded on your property, you can record a release at the Whatcom County Auditor's Office using the city's release of covenant form. [1][2]

Which permit process you'll go through

You need both a land use (ADU) permit and a building permit. The land use permit must be approved before the building permit is issued, but you can submit them at the same time.

• Type I: a staff-level review with no notice to neighbors. The city uses it for "the vast majority of ADUs," even if you ask for minor changes to development standards.
• Type II: used when a project has seven or more homes, including ADUs, under the interim middle housing ordinance. You send a mailing list of owners within 500 feet, the city mails notice, and the decision can be appealed to the Hearing Examiner.
• Type III-A (conditional use): needed when one detached ADU has more than 800 square feet of attached ancillary space, or when two detached ADUs joined together have more than 800 square feet of it or exceed 2,000 gross square feet with it. It requires a neighborhood meeting and a public hearing.

[1][3]

Fees and timeline

Expect an ADU application fee, transportation and park impact fees, and building, plumbing, mechanical and electrical permit fees. The city's ADU homeowner's handbook estimates these impact fees, due when the building permit is issued:

• Park impact fee: $1,470 (half the multifamily rate)
• Transportation impact fee: $1,619.50 with a detached single-family main home, or $902.77 with a duplex, townhouse or other main home (half the main unit's rate)

Impact fees change from year to year, so confirm them with the Permit Center. If the ADU's added fixtures exceed what your water meter can deliver, Public Works may require a larger meter. In addition to the cost of the meter, you then pay water and sewer system development charges, about $7,000, at permit issuance. Some alleys also need improvements if you plan vehicle access from them.

The handbook's estimated timeline is 3 to 5 months to get started, 1 to 2 months for permits under current Type I timelines, and 6 to 12 months for construction. See our ADU cost guide for budgeting. [3]

Renting it out

Every rented ADU in the city must register with Bellingham's Rental Registration program. An attached ADU is exempt from the fees and inspections if there are no other rental units on the property and the owner lives in the main home or the ADU. Detached ADUs are not exempt.

The handbook says short-term rentals, such as Airbnb or VRBO, "are not permitted in D-ADUs in residential single or multiple general use type areas." Where a short-term rental is allowed in a legal ADU, you can get a short-term rental permit for the main home or the ADU, but not both. [1][3]

Pre-approved plans, legalizing a unit and other housing options

Bellingham has one set of registered, pre-approved ADU plans. You still need both permits, but the building permit review "should be faster and less expensive." Plans are bought from the architect of record; ask the Permit Center about site planning first.

If you have an unpermitted ADU, you must apply to legalize it, and units that aren't legalized face enforcement. Existing units must meet the same rules as new ADUs, including building codes.

ADUs aren't the only option. An interim ordinance allows infill housing, up to 6 units per lot, in areas zoned mainly for residential use, except property in the Lake Whatcom Reservoir area. It took effect December 23, 2025, and runs until December 4, 2026, unless the City Council acts. [1][5]

How this fits state law

Washington's ADU law (RCW 36.70A.681) requires cities to allow at least two ADUs per lot in urban growth area zones that allow single-family homes, bars owner-occupancy rules, and says a city can't cap ADU size below 1,000 square feet. Bellingham allows two ADUs, sets a 1,000-square-foot size limit and doesn't require owner occupancy.

Height is a possible conflict. The city's detached ADU limit is 24 feet under height definition #1 or 12 feet under definition #2. State law says a city may not set ADU roof height limits "of less than 24 feet, unless the height limitation that applies to the principal unit is less than 24 feet." The city's height guide says each zone's rules specify which height definition can be used. Ask the Permit Center which limit applies to your lot.

State law says cities must adopt these requirements "to take effect at the same time as the jurisdiction's next periodic comprehensive plan update." For Whatcom County cities, that update was due "On or before December 31, 2025." See our Washington ADU law page. [1][4][6][7][8][9]

Permit steps in Bellingham

1. Check your property

   Confirm your lot allows residential development and isn't in the Lake Whatcom watershed. Review the ADU Quick Guide for size, height and setback limits. [1][2]

2. Talk to the Permit Center

   With a basic design in hand, visit the Permit Center at City Hall, 210 Lottie Street, or contact it at (360) 778-8300 or permits@cob.org. Check with Public Works early about water meter size and alley access. [3][10]

3. Apply for the ADU land use permit

   Submit the ADU permit application. Most ADUs go through a Type I staff review; projects with seven or more homes or extra ancillary space need Type II or Type III-A review. [1]

4. Apply for the building permit

   Submit the building permit application, at the same time or after. Building and land use applications are submitted by email to permits@cob.org. [1][3][10]

5. Pay fees and get your permits

   When review is done and fees are paid, the city issues the ADU permit and then the building permit. Impact fees and any water and sewer charges are due at building permit issuance. [3]

6. Build, pass inspections and register if renting

   Schedule inspections through eTRAKiT, the city's online permitting system. After final approval, register the ADU with the Rental Registration program if you rent it out. [1][3][10]

Pre-approved and sample plans

• Registered pre-approved ADU plan

  One set of registered plans; a land use and building permit are still required, but building permit review should be faster and less expensive. [1]

Who to call

• Bellingham Permit Center

  ADU land use and building permits, feasibility questions

  (360) 778-8300

  City Hall, 210 Lottie Street, Bellingham, WA 98225

  [1][10]

Questions people ask