ADU sizes, types, and rules in Oregon

The three types of ADU

• Detached. A separate building in the yard with its own foundation, entrance, kitchen, and bath.
• Attached. An addition built onto the main house, sharing a wall, with its own outside entrance.
• Interior conversion. A basement, attic, or attached garage converted into a dwelling inside the existing footprint.

How big an ADU can be in Gresham

Gresham allows a detached ADU up to 750 square feet or half the floor area of the main house, whichever is smaller. Attached ADUs and conversions can go up to 900 square feet. The unit also has to meet the setback, height, and lot-coverage rules for the zone, which is what Joel checks on the site visit. Cities revise these numbers from time to time, so treat them as the 2026 rules and let Joel confirm them for your address.

How big an ADU can be in Portland

Portland allows ADUs on most residential lots, and the size limit is tied to the size of the main house, with a cap for detached units. Portland also allows two ADUs on many lots under its residential infill rules. The exact numbers depend on your zone and lot, and Joel confirms them before design starts.

Oregon rules that apply everywhere

• Every city must allow at least one ADU with each detached single-family house in residential zones.
• No extra parking. Cities cannot require an off-street parking space for the ADU.
• No owner-occupancy requirement. You do not have to live in the main house to have an ADU on the lot, which is what lets a parent's unit work when the adult child owns the home, or the other way around.
• The unit must be a complete dwelling with its own kitchen, bathroom, and entrance, built to the residential code and inspected.

Common mistakes when building an ADU

The ADU projects that go wrong usually go wrong in the same places. Starting design before checking setbacks and lot coverage, so the plan has to shrink. Skipping the utility check, then finding the sewer line is on the far side of the house. Building a unit for a parent with a step at the door and a tub in the bath, then paying to change both later. Hiring a designer and a builder separately, with no one responsible for the permit set. GenTerra's process starts with the zoning and the utilities and keeps design, permits, and construction under one person.

The permitting timeline

Design takes a few weeks to a couple of months depending on how quickly decisions get made. City plan review takes several weeks to a few months depending on the queue and how many comment rounds the set needs. Construction of a detached unit takes several months more. Joel gives you a schedule after the site visit. Our permitting page explains what he handles at each step.

Common questions