Bellingham, WA Accessory Structures: ADU Rules (2026)

Bellingham's ADU framework is set in BMC 20.10.036 (Accessory Dwelling Units), updated by Ordinance 2023-08-022 effective August 22, 2023. Bellingham was an early ADU adopter — Ordinance No. 10643 in 1995 allowed attached ADUs (A-ADUs) citywide, Ordinance 2018-05-009 allowed detached ADUs in single-family zones citywide (except in the Lake Whatcom Watershed), and the 2023 update brought the code into pre-emptive compliance with WA HB 1337. Substantive standards under BMC 20.10.036: (A) Number — no more than two ADUs per lot of record in addition to the principal unit; (B) Size — one D-ADU with ancillary space may not exceed 1,800 sq ft (or 2,000 sq ft for two attached D-ADUs) unless approved by the Hearing Examiner via conditional use permit; the floor area of the D-ADU excluding ancillary space (garage, storage) shall not exceed 1,000 sq ft; (C) Height — a D-ADU shall be no higher than 24 ft under BMC 20.08.020 height definition No. 1 or 12 ft under height definition No. 2; (D) Setbacks — 5 ft side and rear yard from property line to foundation, except no setback required when abutting an alley; (E) Parking — eliminated within 0.5 mi of a Whatcom Transportation Authority (WTA) major transit route (high-frequency route with at least 15-minute intervals for at least 5 peak hours weekdays) consistent with RCW 36.70A.681; outside the transit buffer, one off-street parking space typically required; (F) Owner-occupancy — eliminated for new ADUs consistent with RCW 36.70A.681 (Bellingham pre-emptively repealed the owner-occupancy mandate); (G) Design — exterior materials/finish should be compatible with the principal dwelling. WA HB 1337 (2023, codified at RCW 36.70A.680-696) requires GMA-planning cities over 25,000 population (Bellingham, ~95K) to allow at least two ADUs per single-family lot, eliminate owner-occupancy mandates, allow ADUs at least 1,000 sq ft, permit height up to the principal structure or 24 ft, impose no parking minimum within 0.5 mi of major transit, and cap ADU impact fees at 50 percent of the comparable single-family fee. Bellingham's BMC 20.10.036 already meets or exceeds these state minimums. ADU construction requires building permits from the Bellingham Permit Center under the 2018 Washington State Building Code (RCW 19.27 + WAC 51-50 + WAC 51-51). Separately, BMC 20.28 Infill Housing (including the duplex/triplex/fourplex provisions at BMC 20.28.110) was adopted citywide via an interim middle-housing ordinance on May 19, 2025, implementing WA HB 1110 (RCW 36.70A.635). The combination — two ADUs per lot under HB 1337 plus middle-housing forms under HB 1110 — substantially opens single-family lots for additional units, except within the Lake Whatcom Reservoir watershed where additional environmental restrictions remain.