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

Development Services treats an ADU as a residential unit sharing a lot with a single-family dwelling, whether built into the same structure, attached to it, or standing as a separate detached building used as a permanent residence. Inside a designated urban growth area (UGA), a lot may carry two ADUs in one of three configurations: one attached plus one detached, two attached, or two detached (which can be one or two separate structures). Outside a UGA, in the general unincorporated county, a lot is limited to either one ADU inside the existing single-family home plus one detached ADU, or a single detached ADU alone; a second unincorporated-area ADU beyond that combination is not permitted.

Both tracks require the underlying zoning district to allow single-family homes in the first place. 12, and ADUs cannot be subdivided or sold off separately from the principal home. Off-street parking scales with lot size and transit access: no extra parking is required if the parcel sits within a half-mile of a major transit stop, one added space is required on lots under 6,000 square feet, and two spaces are required on larger lots hosting two detached ADUs. Every ADU needs its own Grant County building permit application demonstrating compliance with the section, and the applicant must show all of these configuration, size, and parking requirements are met before the permit is issued.