ADU (accessory dwelling unit) Rules and Regulations in San Juan County, WA (2026)

A San Juan County ADU starts with the parcel’s land-use designation and the kind of dwelling proposed. Internal, attached, and detached ADUs have different approval paths. For detached units outside urban growth areas and activity centers, an annual eligibility process can be a major constraint.

The county rules below focus on the rural detached-unit path. Friday Harbor is a separate town jurisdiction. For property inside an urban growth area, obtain current guidance on state urban ADU protections before applying rural limits to size, count, parking, or ownership.

Detached-unit eligibility

The county’s ADU page directs applicants outside urban growth areas and activity centers through an eligibility process under SJCC 18.40.240(G). A lottery may be needed when applications exceed the available permits. Eligibility approval and a building permit are separate steps.

The April–May 2026 application window has ended. The county page now includes results and information about the next cycle. Its annual notice distinguishes new construction from conversion slots; do not treat a published historical count as permits still available today. Confirm the current window, category, fee, and status directly with Community Development.

An attached or internal ADU does not use the detached-unit lottery path. Zoning, building, health, and other applicable approvals still apply.

Size and parcel limits

The published county code caps an ADU at 1,000 square feet of living area, using the county’s definition of living area. Confirm the measurement against the proposed floor plan.

For detached ADUs outside urban growth areas and activity centers, the published minimum parcel sizes are:

• Five acres in rural districts.
• Ten acres in the agricultural district.
• Twenty acres in the forest district.

These are detached-unit standards, not blanket minimums for an internal apartment. The code states no minimum parcel size for a detached ADU in an urban growth area or activity center, but the permitted-use designation and other requirements still matter. Some districts prohibit ADUs.

Placement and construction order

For a detached ADU, the normal maximum separation is 100 feet between the closest vertical walls of the main home and ADU. The administrator may allow up to 150 feet if the shorter separation would cause greater impact. That longer distance takes the administrator’s approval, so don’t design around it by default.

The code allows either the principal residence or detached ADU to be built first, subject to its conditions. An existing house is therefore not always a prerequisite. The full property plan still needs review.

Water, septic, access, and parking

The published county standards require a shared driveway, water system, and septic/sewer system with the principal residence. They also specify an additional off-street parking space; urban projects need confirmation of applicable state parking protections.

Obtain water availability and septic capacity review early. A shared system must actually support the proposed development. Shoreline, critical-area, and open-space requirements also affect where the home and utilities can go.

For a modular home, Wolf separately checks transport to the island, the delivery route, staging space, and setting equipment. Eligibility does not establish delivery feasibility or installation cost. See island ADUs and cabins for how ferry delivery works.

Ownership and rentals

The published county code requires common ownership of the ADU and principal residence. Confirm the applicable rule for urban property before planning a separate sale; condominium conveyance and land division are distinct questions.

Detached ADUs permitted on or after June 29, 2007 cannot be used as vacation rentals under the county’s published guidance. Do not base the project on short-term rental income without confirming the permitted use and any existing title conditions.

Start with your parcel

Send the address or parcel number, intended use, and whether you are considering a detached home, an attached unit, or a conversion. Keep any eligibility approval, survey, and water/septic records available.

Request a free Wolf property evaluation to start with zoning, utilities, delivery, and model fit.

General planning information. Confirm current adopted requirements and eligibility timing with the county or town before committing to a design.