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

Washington County has two county ADU programs, and its cities have their own. Unincorporated urban neighborhoods follow the county’s urban ADU standards, which can allow two ADUs on one lot. Rural residential land gained its own ADU program in May 2026. Hillsboro, Beaverton, Tigard, Tualatin, Forest Grove, and the other cities each write their own ADU rules.

A Portland or Beaverton mailing address doesn’t tell you which set of rules applies, so start by looking up the parcel.

Who reviews an ADU in Washington County

The county’s urban unincorporated area is organized by community plans, including Aloha-Reedville-Cooper Mountain, Bethany, Bull Mountain, Cedar Hills-Cedar Mill, Metzger-Progress, and Raleigh Hills-Garden Home. Those neighborhoods are not cities. An ADU there goes through the county.

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 the UGB zoned for detached single-unit homes. Local siting and design rules still apply. Those 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 ADUs: CDC 430-2

The county’s urban ADU application instructions (revised July 1, 2026) say urban ADUs are allowed only inside a UGB, in these districts: R-5, R-6, R-9, R-15, R-24, R-25+, TO:R9-12, TO:R12-18, TO:R18-24, FD-20, FD-10, R-6NB, R-9NB, R-15NB, and NMU.

The application’s supplemental form summarizes the standards:

• Up to two urban ADUs with a primary detached dwelling inside the UGB. The form describes one attached ADU and one detached ADU.
• Attached ADU: shares a common wall with the primary dwelling. It can be inside the house or in an addition. The form lists an 800-square-foot limit on habitable floor area, and it also describes an option for an ADU that occupies an entire level of the house. If you are converting a whole floor, ask Current Planning how CDC 430-2 applies.
• Detached ADU: shares no wall, roof, or foundation with the house and is limited to 800 square feet of habitable floor area. It must sit at or behind at least one street-facing wall of the house. It needs a minimum 5-foot side setback, and the rear setback follows the district (or the abutting district, in some cases).
• Height: an urban ADU may not be taller than the primary detached dwelling. An ADU built above a lawful detached accessory building cannot make the combined structure taller than the house.
• Parking and access: no off-street parking is required. An access permit is required for existing and proposed driveways and is part of the ADU approval.
• Order of permits: if the ADU’s building permit issues before the primary dwelling’s, the ADU is treated as the primary dwelling for system development charges and the Transportation Development Tax.

The height rule matters for a factory-built home. Next to a low single-story house, a detached ADU’s roof pitch, floor height, and foundation all count toward the limit. The county asks for elevations of both buildings, so compare them before you choose a model.

After Current Planning approves the urban ADU, you apply for the building permit through Washington County Building Services.

Rural ADUs: CDC 430-3 and Ordinance No. 912A

The Board of County Commissioners unanimously adopted Ordinance No. 912A on April 28, 2026, and it took effect May 28, 2026. The county’s announcement says the ordinance:

• Allows one rural ADU per lot, or conversion of a historic home to a rural historic ADU, in the AF-5, AF-10, and RR-5 rural residential districts.
• Caps a new rural ADU at 900 square feet.
• Applies to most lots of two acres or more in those districts outside a UGB or urban reserve. It excludes groundwater-restricted areas unless an external water provider serves the ADU.
• Requires sufficient fire protection.
• Prohibits vacation occupancy.

The county’s rural ADU application packet (revised June 29, 2026) states that rural ADUs are not allowed in any other land use district. The applicant acknowledges these standards:

• The property is outside the UGB and not in an urban reserve.
• The property is at least two acres.
• There is one lawfully established primary detached dwelling, and it is not subject to a nuisance order or pending nuisance action.
• The ADU is detached, in part of a lawful detached accessory building, or attached to or inside the primary dwelling.
• The ADU has no more than 900 square feet of habitable floor area.
• The ADU is no more than 100 feet from the primary dwelling, measured from the closest portion of each structure.
• The ADU complies with sanitation and wastewater laws, is not used for vacation occupancy, and is not divided onto a separate lot from the primary dwelling.

The packet also lists the district setbacks and height limit:

The packet lists a maximum ADU height of 35 feet. On a corner lot, either the front or the street-side line can serve as the front for setback purposes.

What a rural application includes

The rural packet requires more than drawings:

• Plans: a floor plan and building elevations, plus a site plan showing setbacks, easements, driveways, water bodies, flood and drainage hazard areas, significant natural resources, wells, and septic drainfields. All proposed structures must be at least 10 feet from existing and proposed wells.
• Septic records: a public records request to Washington County Environmental Health for the existing septic system. The packet requires a 10-foot setback from the septic system to proposed structures, and more septic permits and assessments come during the building permit process.
• Water: if a well will serve the house or ADU, the ADU must be at least 10 feet from all wells serving the lot. Otherwise, the packet requires a groundwater-restriction statement from the District 18 Watermaster’s office or a letter from another long-term water provider saying it is willing and able to serve the ADU.
• Fire: written confirmation that a fire protection district is willing and able to serve the property. If the driveway is or will be more than 150 feet long, the Fire Marshal must comment or sign off.
• Service provider letters: from the Washington County Sheriff, the school district, the rural fire district, and the rural water district.
• Access: an access permit for the driveway and a right-of-way permit before the building permit. A second driveway approach must meet sight-distance standards.
• Flood and drainage: a separate alteration application if the ADU or driveway work is in a flood plain or drainage hazard area.

Fees include the rural ADU application fee and a rural development review surcharge for the Oregon Water Resources Department. After land use approval, the building or grading permit brings its own fees, including the Transportation Development Tax and system development charges. Ask the county for current amounts.

City lots follow city code

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

• Hillsboro
• Beaverton
• Tigard
• Tualatin
• Forest Grove

City ADU standards, design review, and fees differ from the county’s and from each other. Utility charges can come from separate providers. Ask for an itemized estimate for your address.

Modular homes in Washington County

Wolf builds modular homes in Battle Ground 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, 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.

A modular home also has to fit the county’s size, height, and placement limits, and it has to be able to reach the pad on a truck.

What Wolf checks first

For a Washington County address, we start with:

1. City, unincorporated urban, or rural?
2. Which district, and is it on the county’s urban or rural ADU list?
3. For urban lots, what is the house’s height, and does a detached ADU fit at or behind a street-facing wall with a 5-foot side setback?
4. For rural lots, is the parcel at least two acres, outside an urban reserve, and able to keep the ADU within 100 feet of the house while meeting district setbacks?
5. What do the septic records show, and how is water supplied?
6. How long is the driveway, and will the fire district confirm service?
7. Which Wolf model fits the size and height limits and can be delivered to the site?

Start with the address

Send us the property address and how you plan to use the ADU. We will confirm the jurisdiction, the district, and the 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 Washington County Development Review/Current Planning or the city that has jurisdiction over your property.