ADU rules in Clackamas County

Clackamas County lists 'Accessory Dwelling Units, subject to Section 839' as an accessory use in every district where a detached single-family home is a primary use: the urban low density districts R-5 through R-30 plus R-2.5, VR-4/5, VR-5/7 and VTH (Table 315-1), the rural residential districts RA-1, RA-2, RR, RRFF-5, FF-10 and FU-10 (Table 316-1), and the Mt. Hood MRR and HR districts (Table 317-1). Only one ADU is allowed per detached single-family or manufactured dwelling inside a UGB or unincorporated community, and one per lot of record outside them. ADUs are prohibited outright in the medium and high density urban districts (PMD, MR-1, MR-2, HDR, VA, SHD, RCHDR) and in the EFU, AG/F and TBR resource zones, and they are not permitted on land in an urban reserve. Two real gates apply: in the Village districts VR-4/5, VR-5/7 and VTH a detached ADU is allowed only above a detached garage, and outside both a UGB and an unincorporated community the lot must be at least two acres and hold exactly one existing house with no guest house and no other dwelling.

Section 839 draws no attached/detached distinction inside a UGB or unincorporated community, and its floor area rules assume an ADU can share a building with the house: all contiguous space in a building counts except the primary dwelling unit itself and unconditioned, unplumbed space reached only through a door. In the VR-4/5, VR-5/7 and VTH districts the code is explicit that an ADU 'either may be located above a detached garage, or it may be integral to a primary dwelling', so an internal or attached unit is the norm there and follows the Table 315-3 standards for primary dwellings. The rural standards in 839.04 likewise say nothing about attachment, and the county's rural ADU FAQ states that the ADU 'may be attached or detached from the primary home'. Basement conversions, additions and garage-top units run through the same building permit path as a free-standing cottage.

Inside a UGB or unincorporated community the cap is 500 square feet in the R-2.5 District and 900 square feet in every other district except the three Village districts. A garage-top ADU is capped at 900 square feet in VR-4/5 and VR-5/7 and at 500 square feet in VTH; an ADU integral to the primary dwelling in those districts has no separate ADU floor area cap and instead follows the Table 315-3 standards for primary dwellings. Outside both a UGB and an unincorporated community the cap is 900 square feet, the unit must sit within 100 feet of the primary dwelling measured structure to structure, and it must meet the primary dwelling setbacks for the zone. Floor area excludes the primary dwelling unit and any unconditioned, unplumbed space behind a door. On height, an ADU in the urban low density districts gets the 35 foot limit that applies to dwellings rather than the 20 foot limit for large accessory buildings, and it is exempt from the accessory building floor space cap in Table 315-2.

Nothing in ZDO Section 839 requires the owner to live on the property, at application or afterward; the word 'owner' does not appear anywhere in the section, and the use tables in Sections 315, 316 and 317 add no such condition. For the county's urban unincorporated areas the ban is statutory: ORS 197A.425 requires counties over 15,000 population to allow at least one ADU per detached single-unit dwelling inside the urban growth boundary subject only to reasonable siting and design regulations, and it says that phrase 'does not include owner-occupancy requirements of either the primary or accessory structure'. Clackamas County, one of the three Portland metropolitan counties, is far above that 15,000 threshold, and the county's own FAQ acknowledges that ADUs "were already allowed inside urban growth boundaries in single-family residential zones" before the 2024 rural expansion. For rural ADUs the county copied the ORS 215.495 conditions into 839.04 and chose not to add an owner-occupancy condition even though ORS 215.495(7) would have permitted one.

Table 1015-2 sets the minimum off-street parking for an accessory dwelling unit at zero in R-5 through R-30, VR-4/5, VR-5/7, VTH, RA-1, RA-2 and 'all other zoning districts', and at one space in the Hoodland Residential (HR) and Mountain Recreational Resort (MRR) districts. Inside the Portland Metro urban growth boundary the zero figure is also compelled by state law, because ORS 197A.425(1)(b)(B) bars a county from requiring additional off-street parking as a condition of ADU approval. HR and MRR cover the Mt. Hood unincorporated communities outside any urban growth boundary, so the statute does not reach them and the one-space requirement there is enforceable. Separately, if the ADU is operated as a short-term rental, County Code 8.10.050(E) requires one off-street space per sleeping area, which attaches to the rental registration rather than to the ADU permit.

ADUs are coded 'A', an accessory use, in Tables 315-1, 316-1 and 317-1, not 'C' for conditional use, so no discretionary hearing before the Hearings Officer or Planning Commission is involved. Design review does not apply either: ZDO 1102.01(B) exempts detached single-family dwellings, manufactured dwellings, prefabricated structures 'and uses accessory to' them. The county's ADU FAQ puts it plainly: 'In most cases, no land use approvals are needed and you can proceed directly to apply for building permits or manufactured dwelling placement permits.' Overlay districts (Habitat Conservation Area, Water Quality Resource Area, Floodplain Management, River and Stream Conservation Area, Historic) can still add review on a specific site, and rural applicants must file fire district access documentation before the onsite wastewater, building or manufactured dwelling placement permit is issued.

No provision limits long-term rental of an ADU and there is no owner-occupancy or primary-residence test. Short-term rental turns entirely on location: an ADU approved under 839.04, meaning one outside both an urban growth boundary and an unincorporated community, may not be short-term rented at all, a ban repeated as note 27 to Table 316-1 and required by ORS 215.495(3). An ADU inside a UGB or an unincorporated community may be short-term rented, because Tables 315-1 and 317-1 list short-term rental in any dwelling unit permitted by the table as a primary use, but County Code Chapter 8.10 requires registration with the county before it can be advertised, a Land Use Compatibility Statement from Planning, renewal every two years, one off-street space per sleeping area, and payment of the 6 percent county transient lodging tax plus a 0.85 percent county STR fee. A rural ADU also cannot be split off and sold separately: ORS 215.495(4)(a) bars the county from approving a partition that puts the house and the ADU on different parcels, and ZDO 839.04(F) prohibits any further dwelling or guest house on a lot that has one.