ADU rules in Bellingham

Detached ADUs (D-ADUs) are a use permitted outright in residential single areas (BMC 20.30.030(A)(7)) and residential multi areas (BMC 20.32.030(A)(8)), subject to the standards in BMC 20.10.036. BMC 20.10.036(B)(5)(a) allows up to two ADUs on a single lot of record 'in any configuration of attached or detached,' so two detached units on one lot are permitted, and they may sit alongside a single-family home, duplex, triplex, fourplex, townhome, or other principal unit. A detached ADU may be converted from an existing structure, including a detached garage, even if that structure violates current setback or lot coverage rules. The one hard exclusion is the Lake Whatcom Reservoir area regulated by Chapter 16.80 BMC, where new ADUs are not allowed at all.

Attached ADUs (A-ADUs), which include basement and other interior conversions as well as additions to the principal dwelling, are likewise a use permitted outright in residential single areas (BMC 20.30.030(A)(6)) and residential multi areas (BMC 20.32.030(A)(7)). The same two-per-lot allowance in BMC 20.10.036(B)(5)(a) covers any mix of attached and detached, so a homeowner may build two attached units, two detached units, or one of each. Under BMC 20.10.036(B)(2) an attached ADU must meet the zoning standards that apply to the principal unit for height, setbacks, floor area and floor area ratio, garage and parking design, accessory buildings, and open space, except where BMC 20.10.036 provides otherwise. The Lake Whatcom Chapter 16.80 BMC exclusion applies to attached units too.

BMC 20.10.036(B)(6)(a) caps any ADU, attached or detached, at 1,000 square feet, and the cap applies per unit rather than per lot, so two 1,000 square foot ADUs are possible. A detached ADU may add up to 800 square feet of attached ancillary space (garage, workshop, garden shed, storage shed) reachable from outside by a person door, but the combined structure is separately limited to 1,800 square feet for one D-ADU and 2,000 square feet for two attached D-ADUs unless the hearing examiner grants a conditional use permit under Chapter 20.16 BMC; even then the ADU floor area itself, excluding ancillary space, cannot exceed 1,000 square feet. Height is the other real limit: a detached ADU may not exceed 24 feet under BMC 20.08.020 height definition No. 1 or 12 feet under height definition No. 2, while an attached ADU takes the principal unit's limit, which is 35 feet and 20 feet respectively in residential single areas.

Bellingham deleted its owner-occupancy rule by Ordinance 2025-12-035, passed December 8, 2025. Until then BMC 20.10.036(B)(4)(b) required an owner to live on any residential-single property containing an ADU and to record an affidavit before the building permit issued, an obligation the code itself said would last only 'until occupancy requirements are preempted by the state law.' The current section's subsection (B)(4) is titled 'Ownership and Land Division' and contains no occupancy requirement at all, matching RCW 36.70A.681(1)(b), which bars a city from requiring the owner of an ADU lot to occupy any unit on it. The city's ADU page states 'Owner occupancy: Not required' for both attached and detached units. Homeowners who recorded a Covenant for Owner Occupancy under the old rule must record a release of covenant at the Whatcom County Auditor's Office to clear title; the covenant does not lapse on its own.

No off-street parking is required for an ADU anywhere in Bellingham. Ordinance 2025-12-035 struck the old requirement of one on-site stall per ADU, and BMC 20.10.036(B)(9) now opens with the conditional 'When parking is provided, the following provisions apply,' making any ADU parking elective. The citywide parking table at BMC 20.12.010(B)(3) lists single-family, duplex/multifamily, hotel and similar uses but has no accessory dwelling unit row, so nothing outside BMC 20.10.036 imposes a number. If an owner does add parking it must meet the design rules: stalls at least eight feet by 18 feet, and no parking in a required front or side street setback except in tandem behind legally established parking. This is more generous than RCW 36.70A.681(2), which only bars parking mandates within half a mile of a major transit stop and caps them elsewhere.

An ADU is a Type I land use decision under BMC 21.10.040(B)(31), which the code defines as 'an administrative review and decision by the director' that is exempt from notice requirements, and BMC 20.10.036(D) routes ADU applications through Chapter 21.10 BMC. In practice an applicant files both a land use permit and a building permit, with the land use approval required before the building permit issues, and the two may be submitted at the same time. Two escalations exist: a Type I decision that needs a SEPA threshold determination becomes Type II (BMC 21.10.040(C)(18)), as does a project of seven or more dwelling units including ADUs under interim Ordinance 2025-12-032; and a detached ADU that exceeds the ancillary-space size ceilings in BMC 20.10.036(B)(6)(b) needs a Type III-A conditional use permit decided by the hearing examiner after a neighborhood meeting and public hearing. Minor modifications to ADU development standards stay within the Type I process.

Nothing in BMC 20.10.036 limits long-term tenancy, and with owner occupancy repealed an owner may rent both the house and the ADUs. Every rented ADU must be enrolled in the Rental Registration and Safety Inspection Program under Chapter 6.15 BMC; the city waives fees and inspection for an attached ADU only when it is the sole rental on the property and the owner lives in the house or the ADU, while detached ADUs are never exempt. Short-term rentals (stays under 30 nights) are governed by BMC 20.10.037: they are allowed in legally established attached ADUs citywide and in detached ADUs in commercial, residential multi and urban village areas, but BMC 20.10.037(B)(5)(c) flatly bars them in detached ADUs in residential single areas until the citywide vacancy rate reaches four percent and the council revisits the question. Only one short-term rental permit is allowed per operator, and on a site with an ADU the permit may cover the principal unit or the ADU but not both; in residential areas the unit must be the operator's primary residence at least 270 days a year and the whole unit may be rented no more than 95 days a year. Separately, BMC 20.10.036(B)(4)(a) allows an ADU to be sold or conveyed as a condominium unit independent of the principal unit.