Accessory Structures in Hillsboro, OR

Every city handles accessory structures a little differently. In Hillsboro, Oregon, there are 3 distinct rules that residents and property owners should be aware of. Some are stricter than what neighboring cities enforce, and others are more relaxed. Here is what you need to know.

Garage Conversions

Hillsboro allows homeowners to convert an existing garage into an accessory dwelling unit (ADU) under Community Development Code § 12.40.100(C)(1), one of three recognized ways to create an ADU alongside adding new floor area or building a detached unit. The converted space must meet the same occupancy, floor-area, parking, height and setback standards in Subsection D that apply to every ADU, and needs approval under § 12.80.170 before construction.

Key details: Governing provision: CDC § 12.40.100(C)(1). ADU cap: One ADU per single detached dwelling/townhouse. Floor area: 250-750 sq ft, or up to 50% for level conversions. Parking: No added off-street parking required. Reduced setback option: 3 feet if 450 sq ft, single story.

An ADU, including one created by garage conversion, cannot be occupied without approval under Section 12.80.170 and compliance with building, fire, health and safety codes required by Subsection D.10. A garage converted to living space without that land-use approval and without a building permit is subject to the City's standard code-compliance and permitting enforcement, and occupancy or utility connections can be withheld by the Building Division until the ADU meets the Subsection D standards.

ADU Rules

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.

Key details: 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.

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.

Shed & Carport Rules

Backyard sheds, detached garages and other accessory structures in Hillsboro follow Community Development Code § 12.40.104: they generally must meet the base zone's setback standards, but a reduced 3-foot side and rear setback is available for structures 450 square feet or smaller, one story tall, with eaves at least 2 feet from the property line, placed behind the home's front building plane, and separated from other buildings by 6 feet.

Key details: Governing provision: CDC § 12.40.104. Default setback: Base zone standard applies. Reduced setback: 3 feet if 450 sq ft or smaller. Height cap for reduced setback: 1 story, max 10 feet. Separation required: 6 feet from other structures.

A shed or detached garage built without meeting the applicable zone's standard setback, or claiming the 3-foot reduced setback without meeting all five specifications in Subsection B.2, is a Development Code violation subject to the City's building-permit and code-compliance process; exceeding the combined lot-coverage cap in Subsection B.3 can likewise block permit issuance until the site plan is revised.

The Bottom Line

Hillsboro's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Hillsboro is broadly strict or permissive.

These rules come from Hillsboro's publicly available municipal code. For complete penalty schedules, exemption details, and answers to common questions, see the individual ordinance pages throughout this guide.