Accessory Structures in Washington County, OR (2026)
4 county-level rules, plus city-specific rules for 2 cities in Washington County, Oregon.
Verified from official government sources
ADU Rules
Washington County allows up to two Urban Accessory Dwelling Units on a single-family lot in the unincorporated urban area, reviewed through a Type I permit under Community Development Code Section 430-2.1. One attached ADU is capped at 800 square feet unless it fills an entire floor of the house or an existing attached garage; detached ADUs also cap at 800 square feet, with a 5-foot minimum side yard setback.
Washington County Urban ADU Zoning Rules
Some RestrictionsWashington County Community Development Code § 430-2.1
A. Number of Urban ADUs. Up to two Urban ADUs are allowed per legal single-family dwelling when the applicable standard below is met: (1) One Urban ADU. When one Urban ADU is provided, the unit may be a detached building, in a portion of a detached accessory building, or attached and/or interior to the primary dwelling. (2) Two Urban ADUs. When two Urban ADUs are provided, one unit must be deta...
ADU Rental Restrictions
Rural accessory dwelling units in unincorporated Washington County cannot be rented out as vacation rentals. CDC §§ 430-3.1.G and 430-4.1.E bar both Rural ADUs and Rural Historic ADUs from 'vacation occupancy' as defined in state law (ORS 90.100), reserving them for longer-term use outside the urban growth boundary.
Washington County Bars ADU Vacation Rentals
Significant RestrictionsWashington County Community Development Code §§ 430-3.1.G, 430-4.1.E
430-3.1 G. A Rural ADU shall not be used for vacation occupancy, as defined in ORS 90.100. ... 430-4.1 E. A Rural Historic ADU shall not be used for vacation occupancy, as defined in ORS 90.100.
Shed & Carport Rules
Detached sheds and other residential accessory buildings in unincorporated Washington County can't cover more than 25% of a required rear yard, face a tiered total-square-footage cap based on lot size, and must meet full zoning setbacks once they reach 120 square feet.
Washington County Accessory Shed Size Rule
Some RestrictionsWashington County Community Development Code § 430-1.1(A)-(B)
430-1.1 Residential: A. Residential accessory uses and structures include, but are not limited to, the following: (1) Treehouses, playhouses and storage sheds greater than 120 square feet in area... B. Development Standards: Detached accessory buildings shall comply with the following, except as modified by other Sections of the Code: (1) Shall not occupy more than 25% of a required rear yard; ...
Garage Conversions
Converting an attached garage into an accessory dwelling unit is the one case in Washington County's ADU code where the usual 800-square-foot size cap does not apply: CDC § 430-2.1.B(1) lets the whole garage floor become an ADU regardless of resulting square footage, provided the ADU still meets Type I permit review.
Washington County Garage-to-ADU Conversion Rule
Some RestrictionsWashington County Community Development Code § 430-2.1.A(1), B(1)
(1) One Urban ADU. When one Urban ADU is provided, the unit may be a detached building, in a portion of a detached accessory building, or attached and/or interior to the primary dwelling. ... B. Floor Area. (1) Attached. An attached Urban ADU shall not exceed 800 square feet of floor area. However, an Urban ADU may occupy up to an entire floor of either the existing primary dwelling or an exist...
Accessory Structures by City in Washington County
2 cities in Washington County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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