Portland ADU (accessory dwelling unit) Rules and Regulations: What You Need to Know Before You Build

Most single-dwelling lots in Portland can have an accessory dwelling unit (ADU), and many can have two. Whether yours can, and how big the ADU can be, depends on the zone, the lot size, the street out front, and the size of the house that’s already there. The rules below are from Portland’s ADU zoning summary and Chapter 33.205 of the Portland Zoning Code.

For how Wolf builds in the city, see ADU builder in Portland. If the parcel isn’t inside Portland, our Multnomah County ADU rules guide explains who reviews it.

What counts as an ADU in Portland

The zoning code defines an ADU as a smaller home on the same lot as the main one, with its own living, cooking, eating, sleeping, and bathroom facilities. It can have a separate exterior entrance or an entrance to a shared interior space that opens to the outside.

You can create one by converting part of the house, adding on to it, converting an existing accessory structure such as a garage, or building a new detached building. “Detached” means stand-alone, not connected to the house by a roof, deck, or other element.

City of Portland or another jurisdiction

A Portland mailing address doesn’t settle who reviews the permit. Check the parcel on PortlandMaps for the zone, overlays, and hazard areas. Some unincorporated areas outside city limits are administered under Portland’s zoning code, and other parcels near the city line belong to Gresham, Milwaukie, or another city, or to Clackamas or Washington County. The Multnomah County guide lists the areas Portland administers.

Which lots can have an ADU

Portland allows a detached ADU in its single-dwelling zones (RF, R20, R10, R7, R5, R2.5), its multi-dwelling zones, its commercial zones, and the Central Employment (EX) zone.

• With a house or manufactured home: one ADU is allowed. In the RF through R2.5 zones the site has to meet the zone’s minimum lot size in Section 33.110.202.
• With an attached house: under PCC 33.205.020.B.1.b, one ADU is allowed if the lot meets the minimum lot area in Table 205-1: 10,000 square feet in R20, 5,000 in R10, 3,500 in R7, 2,500 in R5, and 1,500 in R2.5.
• With a duplex: one detached ADU is allowed in R20 through R2.5 if the site fronts a City-maintained street, meets the Table 205-2 lot size, and isn’t in the Constrained Sites (“z”) overlay. In multi-dwelling, commercial, and EX zones, one ADU is allowed with a duplex regardless of lot size or street.

ADUs can’t be built with floating homes, triplexes, fourplexes, cottage clusters, or multi-dwelling structures.

Can you build two ADUs?

In R20 through R2.5, a site with a house, attached house, or manufactured home can have two ADUs if it:

• Fronts a City-maintained street
• Meets the minimum lot size in Table 205-2: 12,000 square feet in R20, 6,000 in R10, 4,200 in R7, 3,000 in R5, and 1,500 in R2.5
• Isn’t in the Constrained Sites (“z”) overlay

Only one ADU can be attached to the house or to the other ADU, and two ADUs aren’t allowed on a site with a duplex. In multi-dwelling, commercial, and EX zones, two ADUs are allowed with a house regardless of lot size or frontage.

When a lot has two ADUs with a house, at least one unit has to meet the visitability standards in 33.205.040.C.5, covering zero-step access and entrances, the bathroom, living area, and doors.

Size limits

An ADU can be no bigger than 800 square feet or 75 percent of the living area of the main house, whichever is smaller. A house with 1,000 square feet of living area allows a 750-square-foot ADU. A 2,500-square-foot house still caps the ADU at 800.

Living area doesn’t include exterior wall thickness, garages, basement areas with ceilings under 6 feet 8 inches, or areas under 5 feet high or not reachable by a stairway. The limit doesn’t apply to an ADU in the basement of a house that’s at least five years old.

Height, placement, and lot coverage

For a detached ADU:

• Height: up to 20 feet outside the required setbacks, and 15 feet inside them.
• Location: set back at least 40 feet from the front lot line, or placed behind the rear wall of the main house.
• Setbacks: side and rear setbacks start at 5 feet and can be larger depending on the zone. The city’s detached covered accessory structures worksheet shows the rules for your zone.
• Coverage: the ADU can’t cover more ground than the main house, and all detached accessory structures together can’t cover more than 20 percent of the site.
• Floor area: floor area limits apply in every zone except RF. In R10 and R20 they apply only to lots under 10,000 square feet, and in R7 through R2.5 each proposed ADU earns extra floor area.

If a project can’t meet one of these standards, the fallback is an Adjustment Review, a discretionary land use review the city says takes about 8 to 10 weeks with no guarantee of approval.

Design standards for taller ADUs

A detached ADU taller than 15 feet has to match the house or meet these standards:

• Finish materials: match the house, or use wood, composite boards, vinyl, or aluminum, in a shingle pattern or horizontal clapboard or shiplap with boards 6 inches wide or less
• Roof pitch: match the house or be at least 6/12
• Trim: match the house or be 3.5 inches
• Eaves: match the house or project at least one foot
• Windows: match the house or be square or vertical

Properties with a historic designation can have additional design requirements under 33.445 and the Community Design Standards in 33.218.

Parking

Portland requires no extra on-site parking for an ADU. Oregon’s ADU statute, ORS 197A.425, doesn’t let cities require additional off-street parking as a condition of an ADU.

Owner occupancy and short-term rentals

Oregon law also doesn’t allow a city to require the owner to live in the house or the ADU. You can live in either one, or rent both.

Short-term rentals are a separate permit. An ADU can be a Type A accessory short-term rental, renting up to two bedrooms for stays under 30 days, or a Type B rental of three to five bedrooms under the rules in 33.815.040.B.1. If the city waived the ADU’s system development charges, neither is allowed for 10 years.

System development charges and the ADU waiver

New ADUs, and some conversions, can owe system development charges (SDCs) to the transportation, environmental services, and parks bureaus. The Water Bureau charges only if the water meter has to get bigger. SDCs can be a large part of the budget.

The city can waive the SDCs through its ADU SDC waiver program:

• The owner signs a covenant, recorded against the property, that no structure on the property is used as a short-term rental for 10 years. That includes the main house and single bedrooms in it.
• The 10 years start the day of final inspection approval.
• The application goes to the City’s Revenue Division as part of permit review.
• Revoking the covenant, or violating it, costs 150 percent of the current SDCs.

Even with a waiver, a new or larger sewer connection carries its own fees.

Utilities, trees, and hazards

• Water and sewer: the ADU may share the house’s connections or need its own, depending on the fixture count and whether there’s a septic tank. The Water Bureau can confirm your existing service size.
• Trees: trees with trunks larger than 12 inches need protection during construction, and the project’s value can trigger tree planting requirements.
• Slopes and floods: a site with a slope of 20 percent or more, or in a landslide area, needs a geotechnical report. Flood hazard areas have their own limits. PortlandMaps shows both under the Public Safety tab.

The ADU permit process in Portland

1. Research the property. Check the permit history, the zone, and the hazard maps. Free 15-minute appointments with a city planner, a building code reviewer, or a permit technician are available.
2. Prepare the application. Every ADU needs a site plan. A new detached ADU follows the city’s new single-family residence guidelines. The city also offers four free pre-approved detached ADU plan sets.
3. Apply online through Development Hub PDX (DevHub), usually as a single PDF. Building and trade permits (electrical, plumbing, mechanical) can be filed together under one permit number or separately. Electrical work on an ADU has to be done by a licensed electrical contractor, even when the owner does the rest.
4. Answer checksheets. Reviewers send corrections as a checksheet, and review resumes when you respond.
5. Pay fees and receive the permit. The permit isn’t issued until all fees are paid.
6. Build and schedule inspections on DevHub or the automated line at 503-823-7000. Every permit needs a final inspection.

The city assigns one street number to the site, with a letter (A, B, C) for each dwelling unit. The ADU’s address has to be posted at its main entrance and be readable from the street.

Factory-built ADUs in Portland

A Wolf home is modular, not a HUD manufactured home. Oregon’s Building Codes Division Prefabricated Structures Program approves the plans, and each home gets an Oregon insignia of compliance. The insignia isn’t a permit: Portland still issues the installation permit and inspects the foundation, placement, and utility connections. We’ve delivered about three dozen ADUs inside Portland city limits.

Size is where a Portland project starts:

• Model B (307 sq ft) is available only in the City of Portland.
• Model K (432 sq ft) and Models E and F (616 sq ft) are under the 800-square-foot cap.
• Model J is published at 828 square feet, its 23-by-36-foot footprint. Portland measures living area without the exterior walls, which brings it to roughly 760 square feet with 2×6 walls, under the 800-square-foot cap. The open porch doesn’t count. The 75 percent rule still applies, so the main house needs about 1,015 square feet of living area or more. We confirm both numbers against the plan before proposing it.

The Model B starts at $67,500 for the home itself. Permits, SDCs, delivery, installation, and site work are priced after we see the lot. The Oregon ADU cost guide has typical ranges.

Start with your lot

Send us the address. We’ll check the zone, the lot size against Tables 205-1 and 205-2, the street frontage, the house’s living area, and the path a module would take to the pad. Start a free property evaluation.