Clackamas County, OR ADU (accessory dwelling unit) Rules (2026): Urban, Rural, City

Clackamas County has three different ADU situations, and each one has its own rules. A lot inside Oregon City or Happy Valley follows city code. An unincorporated lot inside the Portland-area urban growth boundary (UGB) follows the county’s urban standards. A rural residential property outside the UGB may qualify for the county’s rural ADU path, which started in September 2024 and has its own acreage, size, and distance limits.

A farm-zoned property may not qualify at all. The first step is to identify the jurisdiction and zone for your parcel.

Who reviews an ADU in Clackamas County

For a zoning answer on a specific parcel, contact the county’s Planning and Zoning Division. The county’s ZDO 839 page has the current text of the ADU section.

The statewide rule inside the UGB

ORS 197A.425 requires cities over 2,500 residents and counties over 15,000 to allow at least one ADU for each detached single-unit dwelling in areas inside a UGB zoned for detached single-unit homes. Local siting and design rules still apply. Those local rules cannot require owner occupancy or additional off-street parking. Vacation occupancy can still be regulated. The Oregon ADU rules guide covers the statewide framework.

Unincorporated urban lots: ZDO 839.01

In the county’s urban residential districts, Table 315-1 allows ADUs, subject to Section 839, in the R-5 through R-30 districts, R-2.5, VR-4/5, VR-5/7, and VTH. The table does not allow ADUs in PMD, MR-1, MR-2, HDR, VA, SHD, or RCHDR.

Section 839.01 sets the standards inside a UGB or unincorporated community:

• One ADU per detached single-family dwelling or manufactured dwelling. In the R-2.5 and VTH districts, one ADU per townhouse.
• No ADU in a cottage cluster development or a manufactured dwelling park, and none accessory to a temporary dwelling approved under the county’s temporary permit section.
• Maximum floor area: 500 square feet in R-2.5 and 900 square feet in the other districts, except the VR-4/5, VR-5/7, and VTH districts.
• VR and VTH districts: Sections 839.02 and 839.03 allow the ADU above a detached garage or as part of the primary dwelling. An above-garage ADU can have up to 900 square feet in VR-4/5 and VR-5/7, or 500 square feet in VTH.

How floor area is counted: all contiguous space in the building counts. There are three exceptions: the primary dwelling unit, space separated from the ADU by a wall with no door, and space behind a door that opens only to unconditioned space with no plumbing. An attached, unheated garage with no plumbing, reached through a door, is therefore treated differently from a conditioned bonus room. Settle that on the floor plan before you pick a model.

The county’s rural FAQ describes the urban and community rules as having very few requirements beyond a size limit, usually 900 square feet. Zone setbacks, height, overlays, and utility service still apply to the pad.

Rural residential lots: ZDO 839.04

On August 1, 2024, the Board of County Commissioners adopted amendments allowing ADUs on certain rural residential properties. The amendments took effect September 3, 2024. According to the county’s rural ADU FAQ, rural ADUs are allowed outside UGBs, but not within urban reserves, in these zones:

• RA-2 (rural area residential, 2-acre)
• RRFF-5 (rural residential farm forest, 5-acre)
• FF-10 (farm forest, 10-acre)
• RR (recreational residential)
• FU-10 (future urban, 10-acre)

The FAQ adds RA-1, which occurs in some designated unincorporated communities such as Mulino, Redland, and Colton. Section 839.01 covers ADUs inside an unincorporated community, while 839.04 applies only outside both a UGB and an unincorporated community. Ask staff which set of standards applies to an RA-1 parcel.

ADUs are not allowed in the EFU, AG/F, or TBR zones.

Section 839.04 requires:

• One ADU per lot of record.
• A lot of at least two acres.
• One, and only one, detached single-family dwelling, prefabricated structure, or manufactured dwelling on the lot. No other dwelling can be present, including an RV approved as a second dwelling or a dwelling under a temporary permit, and no guest house.
• Fire protection from a provider whose professionals have the training or certification described in ORS 181A.410.
• No nuisance order or pending nuisance action under ORS 105.550 to 105.600 on the primary dwelling.
• Maximum floor area of 900 square feet, counted the same way as the urban standard.
• Maximum separation of 100 feet from the primary dwelling, measured from the closest portion of each structure.
• Setbacks that meet the zone’s minimums for primary dwellings.
• Fire access: access must be adequate for firefighting equipment, safe evacuation, and staged evacuation areas. The county treats access as adequate when, before it approves the onsite wastewater, building, or manufactured dwelling placement permit, you submit documentation from the fire protection provider acknowledging that access meets the provider’s standards.
• No short-term rental use of the ADU.
• No more dwellings: once the ADU is approved, the lot cannot add another dwelling or a guest house.

The FAQ also says a rural ADU must comply with state laws on water supply, sanitation, and wastewater disposal. At the state level, ORS 215.495 adds limits for wells in groundwater-restricted areas. It also bars dividing the land so the house and ADU end up on separate parcels.

Section 839 was most recently amended by Ordinance ZDO-293, effective September 8, 2026. That package removed references to the State Wildfire Map, which the county says was repealed in 2025. The county has since taken down its hearing summary, so work from the current Section 839 text, not an older printout or handout.

What kind of building can be the ADU

The county’s FAQ says an ADU must be a permitted dwelling: stick-built, a manufactured dwelling, or a prefabricated structure. An RV, a tiny home on wheels, or a similar temporary or mobile structure cannot be an ADU. Using an RV as a second dwelling is a separate program under ZDO Section 847, with its own limits. It is not an ADU approval.

Wolf builds modular homes in Battle Ground to residential building code. For an Oregon destination, the home goes through the Oregon Building Codes Division’s prefabricated structures program. According to the BCD guide for local building departments, the local building department still checks zoning and setbacks and permits the foundation, utility connections, and installation. It issues the certificate of occupancy after inspections. The state insignia is not a certificate of occupancy.

Permits for a rural ADU

The county’s FAQ describes a two-step process:

1. Confirm the rules. Check that the zone allows ADUs and that the property can meet Section 839. Contact Planning and Zoning about overlays, protected areas, or other conditions on the parcel.
2. Apply for permits. In most cases no land use approval is needed. The next filing is a building permit, or a manufactured dwelling placement permit for a manufactured home, through county Building Codes.

Septic is often the deciding question. A system sized for one house may need expansion, or the ADU may need its own system. Fire access documentation also has to be in hand before the county approves the septic, building, or placement permit. Get both answers before you commit to a pad location.

City lots follow city code

If the parcel is inside a city, that city’s rules apply:

• Oregon City
• Lake Oswego
• Milwaukie
• Happy Valley
• Sandy

Each city sets its own ADU standards, design rules, and system development charges (SDCs). Water, sewer, parks, and transportation charges can come from different agencies. Ask for an itemized estimate for your address rather than borrowing a neighbor’s number.

For mountain properties, the county’s FAQ notes that ADUs were already allowed in some designated unincorporated communities on Mt. Hood before the 2024 rural change. On Mt. Hood, confirm whether the parcel is in Clackamas or Hood River County before planning the site logistics.

What Wolf checks first

For a Clackamas County address, we start with:

1. City, unincorporated urban, unincorporated community, or rural?
2. Which zone, and does its use table allow an ADU?
3. For urban lots, which size cap applies (500 or 900 square feet), and how does the floor plan count attached space?
4. For rural lots, is the parcel at least two acres, outside an urban reserve, and occupied by exactly one dwelling with no guest house or RV dwelling?
5. Can the ADU sit within 100 feet of the house and still meet the zone’s primary-dwelling setbacks?
6. Will the fire provider sign off on access, and can septic and water serve a second dwelling?
7. Can a modular home be delivered and set on the pad you want?

Building near Mt. Hood? See mountain cabins and ADUs in the Cascades for snow, the US 26 delivery window, and how other mountain counties compare.

Start with the address

Send us the property address and how you plan to use the ADU. We will confirm the jurisdiction, zone, and likely constraints before recommending a model. Compare Wolf models, review ADU costs and the Oregon ADU cost guide, and read the Oregon ADU rules guide for statewide context. When you are ready, request a property evaluation.

This is general information from Wolf Industries, not legal advice. Confirm current rules, fees, and application requirements with Clackamas County Planning and Zoning or the city that has jurisdiction over your property.