Accessory Structures in Whatcom County, WA (2026)
5 county-level rules, plus city-specific rules for 1 city in Whatcom County, Washington.
Verified from official government sources
ADU Rules
Whatcom County lets an accessory dwelling unit go on most residential and rural lots under WCC § 20.80.910, but the rules split sharply by location: one ADU per lot outside urban growth areas with owner-occupancy and multi-acre lot minimums for a detached unit, versus up to two ADUs with no minimum lot size once the parcel sits inside a designated urban growth area.
Whatcom County ADU Zoning Rules
Some RestrictionsWCC § 20.80.910
Where allowed in the zoning district, all accessory dwelling units shall comply with the following standards and restrictions: ... In addition to an existing or permitted dwelling, there shall be no more than one accessory dwelling unit per lot. 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 ...
ADU Owner Occupancy
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.
Whatcom County ADU Owner-Occupancy Rule
Some RestrictionsWhatcom County Code § 20.80.910(1)(c)
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.
Shed & Carport Rules
A backyard shed in unincorporated Whatcom County skips the building permit only if its footprint stays at or under 200 square feet, under WCC § 15.04.030(C)(1). Cross the size, separation, or height limits and Planning and Development Services requires a full building permit before you build.
Whatcom County Shed Size & Setback Rules
Some RestrictionsWCC § 15.04.030(C)(1)
1. One-story detached, non-habitable accessory structures without basements, used as tool and storage sheds, playhouses, and similar uses, including gazebos, trellis, pergolas and arbors; provided that: a. The building footprint does not exceed 200 square feet (18.6 sq. m) measured outside wall to outside wall; b. Accessory structures maintain a minimum separation of 10 feet [3,048 mm] between ...
Garage Conversions
Inside a Whatcom County urban growth area, WCC § 20.80.910(2)(c)(iii) lets an existing detached garage, barn or other accessory building convert into an accessory dwelling unit even if it does not meet current setback or lot coverage standards. The exception only applies within UGAs; outside them the county's stricter rural ADU standards control instead.
Whatcom County Garage-to-ADU Conversion Rule
Light RestrictionsWhatcom County Code § 20.80.910(2)(c)(iii)
New or existing detached accessory dwelling units may be located on a lot line, if said lot line abuts a public alley; Existing detached structures, including garages, barns and any other accessory buildings, may be converted into accessory dwelling units even if the structure does not meet current setback or lot coverage standards; The maximum size of an accessory dwelling unit shall not excee...
Tiny Homes
Whatcom County recognizes four kinds of tiny home under WCC § 20.97.200: a site-built unit on a permanent foundation, treated exactly like a standard house, and three types of tiny homes on wheels, treated as a mobile home, a park model trailer, or an RV depending on how each is built and used.
Whatcom County Tiny Home Rules
Some RestrictionsWCC § 20.97.200
A "tiny home" is a dwelling unit that is 400 square feet or less in floor area (excluding sleeping lofts). For the purposes of this code there are four types of tiny homes, as described below. (1) Site-Built Tiny Home. A tiny home built on site on a permanent foundation that meets the minimum requirements of the International Residential Code (IRC), including provisions of Appendix Q, and is re...
Accessory Structures by City in Whatcom County
1 cities in Whatcom County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
All Whatcom County ordinances
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