Hillsboro, OR Accessory Structures: ADU Rules (2026)
ADU rules in Hillsboro, OR, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- ADUs per lot
- 1 maximum
- Size range
- 250-750 sq ft
- Off-street parking
- Not required for ADU
- Reduced setback option
- 3 ft, if under 450 sq ft
Summary
Hillsboro allows one accessory dwelling unit per single detached dwelling or townhouse, sized between 250 and 750 square feet, as a use permitted by right under the Community Development Code.
2.Eligibility and Number. A maximum of one ADU is allowed per legal, single detached dwelling or townhouse. If the proposed ADU is on a child lot which resulted from a Middle Housing Land Division, the housing on the lot must be at or above the minimum density for the zone.3.Location. An ADU shall be located on the same lot as the primary single detached dwelling or townhouse.4.Parking. No off-street parking is required for ADUs. Any parking required for the primary dwelling shall be retained or replaced.5.Minimum and Maximum Floor Area. See Section 12.01.500 for information about how to measure floor area. Below are additional standards relative to floor area for ADUs.a.Except as provided in subparagraph c. below, an ADU shall be at least 250 square feet, but not more than 750 square feet in area.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Municipal Code: Supplement 1 | Community Development Code: Supplement 1).
Full Breakdown
The CDC treats ADUs as an outright permitted use citywide, not a conditional use, reflecting Oregon's statewide ADU mandate. Each legal single detached dwelling or townhouse may add one ADU, created by converting existing floor area, adding new floor area, or building a separate structure. Floor area must run 250 to 750 square feet: a basement, attic, or upper-floor conversion can exceed 750 square feet, capped at 50% of the primary dwelling's square footage. No off-street parking is required for the ADU itself. Height, setbacks, and lot coverage otherwise follow the base zone, with a reduced 3-foot side/rear setback available for small detached ADUs meeting specific size and separation limits.
Violations & Fines
Adding a second ADU on one lot, exceeding the 750-square-foot cap without qualifying for the full-level conversion exception, or skipping the required Zoning Review triggers code enforcement and can block occupancy approval.
Frequently Asked Questions
Can I put an ADU on any residential lot in Hillsboro?
Does converting my basement count toward the 750-square-foot cap?
Sources & Official References
Other rules in Hillsboro
How Hillsboro compares: Easiest Cities to Build an ADU·Compare Hillsboro to another location·View the Oregon accessory structures overview
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ADU Rules in Nearby Cities
How other cities in Washington County handle adu rules.