Whatcom County, WA Accessory Structures: ADU Owner Occupancy (2026)
Key Facts
- Owner-occupancy rule
- owner must live in one unit
- Applies
- outside UGAs, plus overlay/rural districts
- Min lot, detached ADU
- 4.5 acres (up to 40 in AG)
- Max ADU size
- 1,248 sq ft (1,748 with credits)
- Compliance tool
- recorded deed restriction
- Governing code
- WCC § 20.80.910(1)(c), (g)
Summary
Outside Whatcom County's urban growth areas, and inside the Lake Whatcom Watershed Overlay and several rural districts even within a UGA, WCC § 20.80.910(1)(c) requires the property owner to live in one of the two dwelling units on a lot with an accessory dwelling unit. The requirement is locked in with a recorded deed restriction before the county will issue a building permit.
The owner(s) of the lot upon which the accessory dwelling unit is located shall occupy as their primary domicile at least one of the dwelling units on that lot.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: rev 4963078; v4 updated 2026-06-23; through 06-23-2026).
Full Breakdown
910(1) governs accessory dwelling units (ADUs) outside designated Urban Growth Areas, and also reaches lots inside a UGA that fall in the Lake Whatcom Watershed Overlay District or the General Commercial, Agriculture, Rural, or Rural Forestry zoning districts. 5 acres for most detached ADUs, rising to 40 acres in the Agriculture District and 20 acres in the Rural Forestry District. 910(1)(g) requires the owner to record a deed restriction with the Whatcom County auditor stating both that a detached ADU cannot be sold or financed apart from the main dwelling, except where the zoning district allows a land division, and that the owner will occupy one of the two units as a primary domicile. 91 WCC. 910(2), the UGA subsection, imposes no equivalent occupancy requirement.
Violations & Fines
Building or occupying an ADU without the owner occupying a unit on the same lot, or without recording the required deed restriction, is a zoning violation under WCC Chapter 20.94: the county can issue a notice of violation and civil penalty of up to $1,000 per day, order a stop-work order carrying its own minimum penalty for noncompliance, and revoke the ADU's building permit under § 20.94.070. The owner may seek remission within 20 days or appeal to the hearing examiner under § 20.94.060.
Frequently Asked Questions
Does the owner have to live on-site to have an ADU in Whatcom County?
How is the ADU owner-occupancy rule enforced in Whatcom County?
Does Whatcom County require owner-occupancy for ADUs inside urban growth areas?
Sources & Official References
Other rules in Whatcom County
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