ADU rules in Hillsboro

A detached backyard ADU is an accessory use permitted by right, with no conditional use hearing, no lot-size gate and no zone-by-zone carve-outs. Every single detached and middle housing residential zone carries the same sentence allowing ADUs accessory to a single detached dwelling or townhouse: R-10, R-8.5, R-7, R-6, R-4.5, SCR-LD (Station Community Residential Low Density), SCR-OTC (Orenco Townsite Conservation) and SCR-DNC (Downtown Neighborhood Conservation). The multi-dwelling zones (Subchapter 12.22) and the mixed-use and urban center zones (Subchapter 12.24) repeat the allowance wherever the primary dwelling is a single detached dwelling or townhouse. One ADU is allowed per legal single detached dwelling or townhouse, and it must sit on the same lot as that dwelling. Ordinance No. 6508 (adopted 11-4-2025) added one condition: an ADU on a child lot created by a Middle Housing Land Division is allowed only if the housing on that lot is at or above the zone's minimum density.

Attached and internal ADUs stand on exactly the same footing as detached ones; Hillsboro writes a single set of ADU standards that covers all three creation methods. Section 12.40.100.C allows an ADU to be created by converting existing floor area in an existing dwelling or accessory structure such as a garage, by adding new floor area to an existing dwelling or accessory structure, or by constructing a separate unit. A unit joined to the house by an architectural or structural connection such as a breezeway counts as attached rather than detached, which matters only because the reduced 3-foot side and rear setback is available to detached units alone. Conversions get two breaks the code gives nothing else: a whole-floor conversion (basement, attic or second story) may exceed the 750 sq ft cap, and a converted ADU is exempt from the rule that an ADU sit behind the front building plane of the house.

The headline cap is tight by Oregon standards: 750 sq ft, with a 250 sq ft floor. Floor area is measured from the interior of the walls, and the calculation may exclude exterior walls, interior staircases, common or shared space between the house and the ADU, interior parking, and unfinished storage not reachable from inside the ADU, so the buildable envelope is somewhat larger than 750 sq ft of footprint. The one escape hatch is a level conversion: an ADU created by converting a basement, attic or second story may occupy that entire floor at any size, subject only to a ceiling of 50% of the primary dwelling's total square footage. Height is not set by the ADU section at all; it defers to the base zone or plan district (for example 2 1/2 stories or 35 feet, whichever is less, in the R-10 zone). The single exception is the setback bonus in Subsection 12.40.100.D.7.c: to drop side and rear setbacks to 3 feet, a detached ADU must be 450 sq ft or smaller in total building area, one story, and no more than 10 feet tall measured at the mid-point between roof peak and eave, with eaves at least 2 feet from the property lines and at least 6 feet of separation from other structures.

No owner-occupancy requirement, at application or ongoing. Section 12.40.100.D sets out the complete list of ADU standards (occupancy restriction, eligibility and number, location, parking, floor area, height, placement, entrances, architectural standards, code compliance) and the word 'owner' does not appear anywhere in the section; there is no deed restriction, no affidavit and no continuing residency condition. State law backs this up: ORS 197A.425(1)(b)(B) provides that reasonable local regulations relating to siting and design 'does not include owner-occupancy requirements of either the primary or accessory structure'. There is one narrow, lawful exception that catches people by surprise. ORS 197A.425(2) preserves local authority over vacation occupancies, and Hillsboro uses it: to run a Partial-dwelling short-term rental the dwelling must be the owner's primary residence and the owner must live on the same property, either in the main house or in a separate dwelling such as the ADU. That condition attaches to the short-term rental permit, not to the ADU itself.

Subsection 12.40.100.D.4 is explicit: 'No off-street parking is required for ADUs.' The only related duty is that parking already required for the primary dwelling be retained or replaced if the ADU displaces it, which is what bites when the ADU is a garage conversion. Hillsboro removed residential parking minimums outright, so Table 12.50.320-1 now publishes only maximums: an ADU may have at most 1 space inside the map's Zone A and 2 spaces in Zone B (Ordinance No. 6488, adopted 12-3-2024, also repealed the old parking exceptions section 12.50.335). ADUs are further exempt from the requirement that development include an ADA-accessible space. This all matches ORS 197A.425(1)(b)(B), which bars cities from requiring additional off-street parking as a condition of ADU approval. Two local wrinkles: in the Orenco Plan District, if the lot abuts a developed alley, any off-street parking you do provide for an ADU must take its vehicular access from the alley; and a Full-dwelling short-term rental (a separate approval) does require one off-street space per two bedrooms.

Administrative and clear-and-objective. Where an ADU is permitted, the only land use approval is a Zoning Review under Section 12.80.170, which runs on the Alternative Review Procedures in Section 12.70.025: no pre-application conference, no neighborhood meeting, no public notice and no opportunity for public comment, with the Community Development Director deciding within 21 days of the applicant completing the upload in the online plan review system. Only the applicant may appeal, within 14 days, and the appeal goes to an outside referee. The approval criteria are checklist items (base zone standards, the ADU special use standards in 12.40.100, and named sections of Subchapter 12.50), and the city publishes an ADU-specific Zoning Review checklist. Building permits may be submitted and reviewed concurrently but cannot be issued before the Zoning Review is approved. ADUs are expressly exempt from Development Review in any zone when no Adjustments to residential design and development standards are requested, and they are also excepted from the rule that any new construction in the SCR-DNC and SCR-OTC conservation zones go through Development Review. The one way an ADU becomes discretionary is if the applicant asks for an Adjustment to a standard: that converts the case into a Type II Development Review with mailed notice and appeal rights.

Nothing in the code conditions who may live in an ADU or on what terms, so renting one month-to-month or by the year needs no approval beyond the ADU itself. The Household Living use category is defined as occupancy 'arranged on a month-to-month basis or for a longer period', which is what pushes anything under 31 days into the separate Short Term Rental special use in Section 12.40.230. If you live on the property and rent the ADU out short-term, that is a Partial-dwelling STR: a Type I permit under Section 12.80.126, the property must be your primary residence proven by tax documents, you must reside on the same property (the code expressly allows the owner to live 'in a separate dwelling on the same property'), only one rental at a time, and only one such permit per owner citywide. If you do not remain on site, it is a Full-dwelling STR, which requires Type II Development Review, is capped at two per owner within the city, requires one off-street space per two bedrooms, and where a property holds more than one dwelling allows only one of them to be rented at a time. Either way a valid City of Hillsboro business license is required and can be revoked by the City Council for cause, with a 60-day waiting period before re-application. Hillsboro levies a 3% transient lodging tax on short-term stays and Washington County levies its own. Separate sale is not possible: an ADU must remain on the same lot as the primary dwelling, and only middle housing (not ADUs) can be split off through a Middle Housing Land Division.