Washington County, OR Accessory Structures: Garage Conversions (2026)
Garage conversion rules in Washington County, OR, sometimes called garage-to-ADU or accessory living unit conversions, govern permits, ceiling height, egress, and parking replacement.
Key Facts
- Attached-garage ADU cap
- None; can use the whole floor
- Detached-garage ADU cap
- 800 sq ft (920 sq ft if accessible)
- Side yard setback
- 5 feet for detached conversions
- Permit type
- Type I development permit
- Governing section
- CDC § 430-2.1.A(1) and B(1)
- Scope
- Unincorporated area only
Summary
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.
(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 existing attached garage even if the floor area of the resulting Urban ADU would exceed 800 square feet. (2) Detached. A detached Urban ADU shall not exceed 800 square feet of floor area.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Community Development Code: Supplement 2 | Code of Ordinances: Supplement 25).
Full Breakdown
Washington County's Community Development Code treats garage conversion as part of its Urban Accessory Dwelling Unit standard rather than as a separate permit category. A(1), a homeowner may locate a single Urban ADU 'in a portion of a detached accessory building' or attach it to and interior to the primary dwelling, covering conversion of either a detached garage or an attached garage into living space. The size limit differs depending on which garage is converted. ' A large attached garage can therefore be converted in full without triggering the 800-square-foot ceiling.
B(2), or 920 square feet if the conversion is built to Uniform Federal Accessibility Standards. Either conversion still needs a Type I development permit, must meet the 5-foot side yard setback that applies to a detached unit, and cannot exceed the height of the primary dwelling. This rule only reaches unincorporated county land; a garage conversion inside Hillsboro, Beaverton or another incorporated city in the county runs through that city's own code instead.
Violations & Fines
Converting a garage into an ADU without an approved Type I permit violates CDC § 215-1, which prohibits any construction, alteration or land use that conflicts with the Development Code. The county can deny or condition a building permit application where it finds an existing violation on the property under § 215-2, and confirmed violations are prosecuted under Washington County Code Chapter 1.14 per § 215-3, through a citation heard by a county hearings officer or the circuit, district or justice court.
Frequently Asked Questions
Can I convert my attached garage into an ADU in Washington County?
Does converting a detached garage get the same size break?
What setback applies to a converted detached garage?
Sources & Official References
Other rules in Washington County
Compare Washington County to another location·View the Oregon accessory structures overview
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