Multnomah County, OR ADU (accessory dwelling unit) Rules (2026): City or County Review?

“Multnomah County” is on the tax statement for an inner-east Portland bungalow, a Gresham ranch house, and a rural property out past Corbett. Those three owners do not file ADUs with the same office, and they do not work from the same rules. A home inside Portland, Gresham, or another city is a city ADU project. The county’s own zoning code, which the county describes as the zoning for its rural areas, covers only the unincorporated land that no city administers, and its ADU allowance there is narrow.

Start with who reviews your address. Size limits, fees, and model choice come after that.

Who reviews an ADU in Multnomah County

The county’s urban pocket page lists the unincorporated areas that neighboring cities administer through intergovernmental agreements (IGAs): Portland’s Dunthorpe, Pleasant Valley, Southern Border, West County, and West Hayden Island areas; Gresham’s 162nd area; and Troutdale’s Sundial and Troutdale areas. Those properties carry city zoning even though they sit outside city limits. The City of Portland administers its zoning code for specific unincorporated areas and requires a certification of service availability for projects there.

A mailing address does not settle any of this. Pull the parcel on the county’s planning map or ask the county planner which code applies.

Building permits can come from a different office

Zoning and building permits do not always come from the same office. The county’s service-provider list sends building permits to three cities, including for some properties outside their limits:

• Portland: properties in Portland, west and north of Portland, on Jenne Lane, and off 162nd south of Foster.
• Gresham: properties in Gresham, in Pleasant Valley, and east of Gresham, except in Fairview, Troutdale, or Wood Village.
• Troutdale: properties in Troutdale, on Interlachen Lane, and on Blue Lake Road.

Fairview and Wood Village handle their own properties. A rural lot east of Gresham can therefore have county zoning and a Gresham building permit. Plan for both reviews from the start.

The statewide rule inside the urban growth boundary

ORS 197A.425 requires cities with more than 2,500 residents and counties with more than 15,000 to allow at least one ADU for each detached single-unit dwelling in areas inside the urban growth boundary (UGB) zoned for detached single-unit homes. Local siting and design rules still apply. The statute bars owner-occupancy requirements and requirements for additional off-street parking. It still lets local governments regulate vacation occupancy. Our Oregon ADU rules guide covers the statewide framework.

Outside the UGB, ORS 215.495 lets a county adopt a rural residential ADU program. Adoption is optional, and a county may impose stricter rules.

Unincorporated Multnomah County: what Chapter 39 says

The county posts its zoning code, Chapter 39, on its Zoning Codes page. The code lists ADUs in these zones: Multiple Use Agriculture (MUA-20), Rural Residential (RR), Orient Rural Center Residential (OR), Orient Commercial-Industrial (OCI), Urban Future (UF-20), and Urban Low Density Residential (LR-5, LR-7, and LR-10).

In every one of those zones, the first standard is that the ADU must be sited entirely inside the urban growth boundary. The code does not list ADUs in the exclusive farm use, commercial forest use, or rural center zones. Chapter 39 does not include a rural ADU program under ORS 215.495. A county-zoned property outside the UGB therefore has no ADU option under the posted code, even in a rural residential zone. If you own rural land in the West of Sandy River or East of Sandy River planning areas, ask Land Use Planning whether an amendment has changed that before you spend money on design.

Where the county’s code does allow an ADU, the listed standards include:

• Size: floor area no more than 800 square feet or 75 percent of the house’s floor area, whichever is less. A 900-square-foot house caps the ADU at 675 square feet.
• Placement: the ADU is attached to or inside the house; inside an accessory building that existed when the ordinance took effect; or detached (or inside a newer accessory building) at least 7 feet and no more than 20 feet from the house, measured between the closest exterior walls. The code allows no variance to those distances.
• Driveway: the ADU uses the house’s lawfully established driveway entrance, although the driveway may be extended. Transportation impacts are mitigated under the County Road Rules, and the shared-driveway rule has no variance.
• Entrance: an attached or interior ADU needs at least one separate exterior door.
• Building type: RVs, park models, yurts, and structures unable to meet construction or installation standards are not allowed as ADUs.
• Rental use: short-term rental of the ADU (fee-based occupancy under 30 consecutive days) is prohibited. The owner records a covenant that runs until the property is annexed into a city. Month-to-month long-term rental is not short-term rental.
• Other limits: the ADU cannot be accessory to a health hardship dwelling or other temporary dwelling. Buildings accessory to the ADU are limited to 400 square feet of combined footprint.

The 7-to-20-foot window matters for a factory-built home. A detached unit that close to the house affects the crane swing, fire separation, eaves, and utility runs. Check it on the site plan before you choose a model.

Properties in the Columbia River Gorge National Scenic Area follow a separate chapter, Chapter 38. Ask Land Use Planning about any second-dwelling option there before assuming an ADU is possible.

Portland, Gresham, and the other cities

Each city writes and amends its own ADU code.

• Portland: the city offers an ADU SDC waiver. Its recorded covenant prohibits short-term rentals across the whole property, including the main house and individual bedrooms, for 10 years from final inspection approval. The separate temporary housing exemption under Portland Code 17.14.070 J expressly excludes ADUs. Wolf’s Model B is offered for City of Portland projects only.
• Gresham: Gresham’s own development code and fees apply.
• Troutdale, Fairview, Wood Village, and Maywood Park: each has its own planning office. Confirm the ADU standards, design rules, and system development charges (SDCs) with that city.

SDCs vary by city and service district. Portland’s waiver does not tell you what a Gresham or Troutdale lot will owe. Ask for an itemized estimate before committing to a design.

Septic, water, access, and fire

• Septic: the county’s service-provider list says the City of Portland manages on-site sewage disposal and septic systems throughout the county. An existing system sized for one house may not serve a second dwelling. Confirm capacity before locating the ADU.
• Road access: Multnomah County Transportation Planning and Development Review handles access permits on county roads in unincorporated areas. ODOT handles state highways, such as US 30 and Cornelius Pass Road on the west side and the Historic Columbia River Highway and I-84 on the east side.
• Fire service: unincorporated areas are served by different providers, including Gresham Fire/Multnomah County Rural Fire Protection District #10, Corbett Fire District #14, Sauvie Island Fire District #30, Scappoose Fire District, and Tualatin Valley Fire & Rescue. Driveway length and turnaround questions go to the provider that serves the parcel.

Property taxes after an ADU

Multnomah County’s Assessment page on ADUs says the county assesses and taxes an ADU like any other new improvement under OAR 150-308-200. After construction starts or finishes, an appraiser reviews the whole property and the added value is assessed. The county notes that Oregon’s property tax limits make the exact increase hard to predict. Budget for it separately from construction.

Modular homes in Multnomah County

Wolf builds modular homes in its Battle Ground factory to residential building code. They are not HUD code (the federal standard for manufactured homes) manufactured homes. 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, that program approval covers the building from the floor to the roof. The local building department still checks planning and zoning, setbacks, the foundation, utility connections, and installation, then issues the certificate of occupancy. The state insignia is not a certificate of occupancy, and state approval of the building has no bearing on whether zoning allows an ADU on your lot.

What Wolf checks first

For a Multnomah County address, we start with:

1. Is the parcel inside a city, in a city-administered IGA area, or county-zoned?
2. If county-zoned, is it inside the UGB, and which Chapter 39 zone applies?
3. Which office issues the building permit?
4. What is the house’s floor area, and what does the 800-square-foot or 75 percent rule allow?
5. For a detached unit, can it sit 7 to 20 feet from the house with workable delivery and crane access?
6. Is the lot on sewer or septic, and can the utilities serve a second dwelling?
7. Is the intended use long-term, family, or short-term? The covenants and waivers depend on the answer.

Start with the address

Send us the property address and how you plan to use the ADU. We will confirm the jurisdiction, the zone, and whether a model fits before anyone talks finishes. 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 Multnomah County Land Use Planning or the city that has jurisdiction over your property.