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.

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.

Garage Conversions

Hillsboro lets a homeowner convert an existing garage into an accessory dwelling unit under Community Development Code § 12.40.100(C)(1). The unit must measure at least 250 and no more than 750 square feet, only one ADU is allowed per single detached dwelling or townhouse, and the city requires no off-street parking for the ADU itself. Zoning Review under Section 12.80.170 applies.

Key details: ADU floor area: At least 250 and not more than 750 square feet. Whole-floor conversion cap: No more than 50% of the primary dwelling's square footage. Off-street parking: None required for the ADU; main house parking retained or replaced. Number allowed: One ADU per legal single detached dwelling or townhouse. Entry porch: Recessed at least 2 feet deep and 5 feet wide.

Section 12.40.100 sets conditions for approval rather than a fine schedule, and it prints no penalty amount. A garage conversion that falls outside 250 to 750 square feet, loses required parking for the main house without replacing it, adds a second ADU to the same dwelling, or ignores the entrance, lighting and glazing standards cannot be approved through Zoning Review under Section 12.80.170. Work that fails the building, fire, health and safety codes under subsection (D)(10) is handled under those codes. An applicant who cannot meet a numeric standard can request a Variance or Adjustment under Section 12.80.150.

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.