The short answer: Oregon ADU rules date back to the first state law requiring cities to allow ADUs. Under ORS 197A.425, cities with more than 2,500 residents and counties with more than 15,000 must allow at least one ADU for each detached single-family dwelling inside the urban growth boundary. They cannot require owner occupancy or additional off-street parking. Portland goes further than the state minimum: up to two ADUs on many lots, no extra parking, and a size cap of 75% of the main house or 800 sq ft, whichever is smaller.
Last verified: October 2, 2026 ยท Written by the ADU Scope editorial team
Key fact: The state floor applies only to areas inside an urban growth boundary (UGB) that are zoned for detached single-family homes. Rural land outside a UGB follows county rules and, in many cases, farm and forest land-use restrictions.
What Oregon ADU Rules Require
ORS 197A.425 sets a short list of obligations. Each applies to a city of more than 2,500 people or a county of more than 15,000 people, and only inside the urban growth boundary.
| Topic | What the statute says |
|---|---|
| Who must comply | Cities over 2,500 residents; counties over 15,000 |
| Where | Areas inside the UGB zoned for detached single-family dwellings |
| How many | At least one ADU for each detached single-family dwelling |
| Types | Interior, attached or detached structures used in connection with, or accessory to, a single-family dwelling |
| Owner occupancy | Cannot be required for the ADU or the main dwelling |
| Extra parking | Cannot be required |
| Local standards | Only “reasonable local regulations relating to siting and design” |
The phrase “reasonable local regulations relating to siting and design” is where most of the variation between cities lives. Cities can set height limits, setbacks, maximum size and design standards, as long as those do not amount to a ban. Two cities that both comply with the state floor can still have very different ADU codes.
The statute also lets local governments regulate vacation occupancy separately. A city may require owner occupancy or parking for short-term vacation use even though it cannot require them for ordinary long-term housing.
Tip: If your town has fewer than 2,500 residents, the state mandate does not apply to it. Check the local code, because a small city may still allow ADUs voluntarily.
Why the Statewide Floor in Oregon ADU Rules Is Only the Starting Point
The state mandate guarantees one ADU, not a particular size, height or design. That leaves room for local rules that are legal but restrictive in practice. A city could, for example, set a modest maximum floor area, require detached ADUs to sit behind the main house, or apply design standards that raise costs.
What to check in your local code
- Maximum floor area and whether it is a fixed number or a share of the main house.
- Maximum height, which often differs inside and outside setbacks.
- Setbacks and any requirement that a detached ADU sit behind the main structure.
- Design standards such as matching materials or roof pitch.
- System development charges (SDCs) for water, sewer, parks and transportation, which cities set themselves.
This is why a statewide article can only take you so far. The reliable way to know what you can build is to read the ADU section of your city’s zoning code and then confirm with a planner.
Portland: How Many ADUs You Can Build
Portland’s zoning code distinguishes between single-dwelling zones and multi-dwelling or commercial zones.
| Zone type | ADUs allowed |
|---|---|
| Single-dwelling zones, with a house, attached house or manufactured home | One ADU; two ADUs if the site has City-maintained street frontage, meets the minimum lot size, and is not in a Constrained Sites overlay |
| Multi-dwelling, commercial and central employment zones | Two ADUs on a site with a house or attached house, regardless of site size or street frontage (only one can be attached) |
| Sites with a duplex | One detached ADU only |
The second ADU in single-dwelling zones depends on three conditions that are easy to overlook: street frontage maintained by the City, a minimum lot size, and no Constrained Sites overlay. Many lots on unimproved or privately maintained streets fail the first test.
Attention: Do not assume the two-ADU allowance applies to your lot. Confirm frontage, lot size and overlay status on the City’s zoning map before paying for design work.
Oregon ADU Rules in Portland: Size, Height and Placement
Portland caps ADU size at 75% of the living area of the main structure or 800 sq ft, whichever is smaller. There is an exception: no size limit applies when the ADU occupies the basement of a primary structure that is at least five years old. That exception makes basement conversions the most flexible option on many older Portland lots.
Height and setbacks
- Height outside required setbacks: 20 feet maximum.
- Height inside required setbacks: 15 feet maximum.
- Detached ADU location: set back 40 feet from the front lot line, or located behind the rear wall of the main structure.
Because the size cap is the lesser of two numbers, a small house limits a new ADU even if the lot is large. A 900 sq ft bungalow, for example, caps a new ADU at 675 sq ft (75% of 900), while a 2,000 sq ft house caps it at 800 sq ft.
Oregon ADU Rules on Parking, Owner Occupancy and Short-Term Rentals in Portland
Portland states that no extra on-site parking is needed for an ADU. That matches the state rule against requiring additional off-street parking. The City’s ADU page does not specify an owner-occupancy requirement, which is consistent with the statute barring one.
Short-term rentals are handled separately. Portland defines Type A short-term rentals as up to two bedrooms with stays under 30 consecutive days, and Type B as three to five bedrooms requiring conditional use approval. The City also notes that SDC waivers restrict short-term rental use for 10 years, so an ADU that benefits from a waiver cannot simply become an Airbnb.
Tip: If you plan to use the ADU for short stays, read the conditions of any SDC waiver before you apply. The restriction is long and tied to the waiver, not just to general zoning.
System Development Charges and Fees in Portland
Portland’s temporary SDC exemption for new housing, running from August 15, 2025 through September 30, 2028, does not apply to ADUs. The City says a separate SDC waiver program exists for accessory dwelling units. Because eligibility, conditions and funding for that program can change, confirm the current terms with Portland Permitting & Development before budgeting.
Two practical points follow. First, do not copy figures from older articles that quote SDC waiver amounts; they may be outdated. Second, include SDCs in your budget whether or not you expect a waiver, since you can only count on a waiver after the City confirms it in writing.
Frequently Asked Questions
Does Oregon require cities to allow ADUs?
Yes, for cities over 2,500 residents and counties over 15,000, inside the urban growth boundary, on lots zoned for detached single-family homes.
Can a city require me to live on the property?
No. ORS 197A.425 excludes owner-occupancy requirements from reasonable local regulation, except for vacation occupancy.
Do I need extra parking for an ADU?
No. State law bars additional off-street parking requirements, and Portland states none is needed.
How many ADUs can I build in Portland?
One in most single-dwelling lots, two if you meet frontage, lot size and overlay conditions, and two in multi-dwelling and commercial zones.
How big can a Portland ADU be?
75% of the main house’s living area or 800 sq ft, whichever is smaller, with no limit for a basement ADU in a house at least five years old.
Next Steps
- Washington State ADU Law: What Homeowners Can Build
- ADU Laws by State: Where You Can Build in 2026
- ADU Impact Fees: When You Pay Them and How to Avoid Them
- ADU Setback Requirements Explained: California, Washington, Colorado and Portland
- Can My HOA Block an ADU? Rules by State
- Colorado ADU Law 2026: What HB24-1152 Means for Homeowners
General information, not legal advice. Local rules vary and change; confirm with your planning department.
Sources: ORS 197A.425 ยท City of Portland: ADU zoning requirements ยท City of Portland: Temporary SDC exemptions for new housing units