Portland’s zoning rules distinguish an ADU’s living area from the height, coverage, location, and design standards that apply to a detached unit. The City’s summary directs readers to the property-specific zoning rules and Chapter 33.205 for the controlling provisions. The figures below describe selected current standards; a base zone, overlay, project form, or other applicable rule can change what fits on a particular site.
Calculate the ordinary living-area cap
The City summarizes the ordinary ADU maximum as the lesser of 75% of the main structure’s living area or 800 square feet. In its living-area calculation, the City says not to include exterior-wall thickness, garage areas, basements with less than 6 feet 8 inches of ceiling height, or areas with less than 5 feet of floor-to-ceiling height or without stair access.
The City states that this ADU size limit does not apply when the unit is in the basement of a primary structure that is at least five years old. That exception concerns the stated size limit; it does not remove other zoning, building, or permit requirements. Match the proposed space to the City’s definition and verify the structure’s age and permit record before relying on the exception.
Check the detached-unit envelope
For a detached ADU, the City’s current summary lists a maximum height of 20 feet outside required setbacks and 15 feet inside them. It also describes two location alternatives: at least 40 feet from the front lot line, or behind the rear wall of the main structure. The required setbacks and measurement points still depend on the code and the site’s zone.
The same summary says a detached ADU’s building coverage cannot exceed the primary structure’s coverage and that all detached accessory structures together must not cover more than 20% of the site. A detached ADU taller than 15 feet also has exterior-design standards. Check Chapter 33.205 and the applicable base-zone rules for the complete requirements; the City’s detached covered-accessory-structure worksheet is intended to show which rules apply to a project.
Recheck the effective code date
Portland lists its current printable Title 33 code as effective July 1, 2026. The City also lists Ordinance 192213, a package of zoning-code amendments, with an effective date of November 1, 2026. The City announcement does not identify which ADU provisions, if any, the package changes. This page reflects the standards shown by the City before that effective date. For an application on or after November 1, check the adopted ordinance, current Chapter 33.205, and any transition rule before relying on these measurements.
Dimensional compliance does not establish building-code or fire compliance, utility capacity, historic approval, or a permit result. Confirm the property’s base zone and overlays, project form, survey measurements, current code text, and any site-specific review with Portland’s planning and building authorities.