Island County, WA Accessory Structures: ADU Rules (2026)

I treats ADUs as a permitted, ministerial (Type I) use rather than a discretionary permit. A property owner may add only one ADU, attached or detached, per single-family dwelling; a second unit is not allowed no matter how the parcel is zoned among the five eligible rural zones. A detached ADU cannot exceed 1,200 square feet of gross floor area, excluding garage or shop space that is not living space, and must share its driveway access with the main house rather than opening onto a separate curb cut.

It must sit within 100 feet of the primary residence or wherever the environmental impact is least while preserving rural character. Planning and Community Development limits the whole county to 35 detached-ADU building permits per calendar year for properties outside urban growth areas, and no applicant may receive more than one detached-ADU permit in a single year. Attached ADUs, created by converting existing floor area, adding square footage, or building into new construction plans, are capped at 1,000 interior square feet. Neither type may host a home occupation or home industry, and detached units cannot be split off and sold separately unless the remaining lot still meets base density and other county requirements. 07D before Planning and Community Development will sign off.