Hillsboro, OR Accessory Structures: Garage Conversions (2026)

Hillsboro treats a garage conversion as one of three ways to create an accessory dwelling unit. Section 12.40.100(C)(1) names it directly: converting existing floor area in an existing dwelling or accessory structure such as a garage. ADUs are permitted by right as an accessory use to single detached dwellings and townhouses under Section 12.21.140(A), but each one is reviewed and approved under Section 12.80.170, and an applicant who cannot meet a standard can ask for a Variance or Adjustment under Section 12.80.150.

Size is the first test. Under § 12.40.100(D)(5)(a) the ADU must be at least 250 square feet and not more than 750 square feet. Exterior walls, interior staircases, space shared with the main house, interior parking and unfinished storage that cannot be reached from inside the ADU can be left out of the measurement. A conversion of a whole level or floor of the primary dwelling, such as a basement, attic or second story, can fill that entire level even above 750 square feet, but the unit cannot exceed 50% of the primary dwelling's total square footage.

Parking works in the owner's favor. Subsection (D)(4) says no off-street parking is required for ADUs, but any parking required for the primary dwelling must be retained or replaced. Only one ADU is allowed per legal single detached dwelling or townhouse, it must sit on the same lot as that home, and it carries the maximum occupancy of a dwelling unit defined in Section 12.01.500.

Placement rules are softer for conversions. An ADU must normally sit behind the front building plane of the main house, but ADUs created by converting existing floor area are excepted, so a street-facing garage can still qualify. Height follows the zone, and no ADU can sit in a public or private utility or access easement.

The converted unit needs its own entrance with a recessed entry or porch at least 2 feet deep and 5 feet wide, entrance lighting that does not exceed 2 lumens or glare onto neighbors, and a sidelight, glass door or window. Architecture must include 2 or more articulation or detail elements from Section 12.50.710 on every façade. Glazing facing an abutting residential property within 20 feet is restricted unless windows sit in the upper one-third of each story or a screen is installed. The screen must be at least 6 feet tall, run the full width of the ADU wall plus 3 lineal feet, and use shrubs at least 5 feet high when planted. The ADU must also meet building, fire, health and safety codes.