ADU rules in Oregon City

One ADU is allowed per detached single-family residential unit, and a detached ADU is one of the two types the code expressly recognizes (a converted detached garage, a new backyard structure, or an old house kept as the ADU while a new principal dwelling is built). ADUs reach the low and medium density zones (R-10, R-8, R-6, R-5, R-3.5) through the permitted use 'accessory uses, buildings and dwellings' and the same phrase in the HC Historic Commercial district. They may be added to an existing detached single-family home or built at the same time as a new one, and they do not count against the zone's minimum or maximum density. Detached units must sit behind the front building line of the principal dwelling or at least 40 feet back, whichever is less, and meet the base zone's side and rear setbacks.

Attached and internal ADUs are the second recognized type: converted living space, an attached garage, a basement or attic, an addition to the existing dwelling, or a combination. They are reviewed under the same Type I process and the same 800 sq ft / 60 percent size cap as detached units, except that converting an existing basement is exempt from the size cap when no new floor area is added. Additions may not encroach into the base zone's minimum setbacks, though stairs or ramps may sit in a setback if there is no other way to reach the unit. The city's ADU page lists an addition and an internal conversion as two of the three main ADU forms.

The size rule is a 'whichever is less' test, so on a 1,000 sq ft house the cap is 600 sq ft, not 800. Converting an existing basement escapes the cap entirely as long as no new floor area is added with the conversion. Detached units may rise to the greater of 20 feet or the height of the principal dwelling; attached ADUs simply live within the base zone's height limit. Base zone lot coverage still binds the house and ADU together, which the city's January 2025 application packet states as 45 percent in R-10, R-8 and R-6, 60 percent in R-5 and 65 percent in R-3.5.

Nothing in OCMC 17.20.010 conditions an ADU on the owner living on the property, and the city's January 2025 application packet asks no occupancy question. State law forecloses the requirement anyway: ORS 197A.425(1)(b)(B) (the recodified SB 1051 / HB 2001 mandate, formerly ORS 197.312(5) and (6)) provides that reasonable local siting and design regulations 'does not include owner-occupancy requirements of either the primary or accessory structure'. The one place occupancy does bite is short-term rental use: ORS 197A.425(2) preserves local authority over vacation occupancies, and Oregon City's bed and breakfast conditional use standards require the owner or operator to live in the unit or next door. Renting the ADU long term or living in the ADU and renting out the main house are both fine.

OCMC 17.20.010.D.7 states flatly that no off-street parking is required for an ADU, and Chapter 17.52 (off-street parking) does not apply to single-family detached residential development in the first place. ORS 197A.425(1)(b)(B) independently bars the city from requiring additional off-street parking as a condition of ADU approval. Watch out for the city's own ADU guide PDF, produced in September 2019 and still linked from the ADU page, which says one off-street space is required; that statement was overtaken by the later amendments to 17.20.010 and by state law. The 2025 application packet asks only about driveway count and width, not parking spaces.

ADU applications are processed as a Type I minor site plan and design review under OCMC 17.62.035, filed concurrently with the building permit. The code defines Type I as involving no discretion, and 17.62.035.A.2.z lists accessory dwelling units among the projects eligible for it; the city's ADU page says staff typically review these within two weeks with no hearing and no neighbor notice. The discretionary path opens only when something else pulls the project out of Type I: a variance, or a site in the historic, flood management, geologic hazard, natural resource or Willamette River Greenway overlays, or a nonconforming lot, site or structure, which the city routes to Type II or Type III review starting with a pre-application conference. New construction inside a historic or conservation district also needs a certificate of appropriateness from the historic review board.

Oregon City draws the line at 30 days. OCMC 17.04.145 defines bed and breakfast inns and boardinghouses as buildings providing overnight accommodations to the public for fewer than 30 consecutive days, and that use is only a conditional use in every residential zone (17.08.025.B, 17.10.025.B, 17.12.025.B), meaning a Type III hearing before the Planning Commission. The conditional use standards cap guest rooms at four in a residential zone and require that 'the owner/operators shall reside in the bed and breakfast inn, or in a residence adjacent to the bed and breakfast inn' (17.56.040.G.2). The city's short-term rental page adds that a business license is required and that a local transient room tax will likely apply, while confirming that most residences may be rented for 30 days or more with no special permitting. Nothing in the code caps rent, requires registration of a long-term ADU tenancy, or bars renting the ADU while the owner lives elsewhere.