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Development Services

From Concept to Construction

Phone: 503-823-7300

TTY : 503-823-6868

1900 SW 4th Ave, Portland, OR 97201

Accessory Dwelling Units (ADUs)

You will find information on this page about adding an accessory dwelling unit to a property. Every dwelling unit functions as a separate unit. They are commonly known as "mother-in-law apartments", "granny-flats" or studio apartments.


Applying For My Permit

The following information must be submitted when applying for a permit to build an ADU. Since every project is unique, there may be some situations where you will be asked to provide additional information. 

To submit for a permit, bring these forms, and four (4) copies of site, architectural, and structural plans (for the area of proposed work and areas affected by such work ) along with intake fees to the Development Services Center.

If you are hiring sub-contractors to do work on your project, and know who they are when applying, they will be listed on your permit at that time. If you don't know when you apply for your permit, they can be added after issuance. Sub contractors you hire must be registered with the State of Oregon and carry a current CCB license.

Things to Consider


(1) Design Standards

There are a variety of design and use standards which apply to the creation of an ADU. These include limiting the overall number of residents in both units, limitation with respect to home occupations, the location of entrance doors and size of the ADU. Where an ADU is created in a new or existing detached structure or by the addition to a house, there are additional standards regulating the exterior appearance and materials of the ADU. These additional standards include provisions for the height, roof pitch, trim, eaves and windows.

  • Size of ADU: The ADU may be no more than 75% of the total living area of the house or a maximum of 800 square feet, whichever is less.
  • Entrance Location: Only one entrance can be located on the facade facing a street. If an existing building has more than one street facing entrance, then each unit can have a street facing entrance. A secondary entrance to each unit can face a street where there is not access to the ground floor but the door provides access to a balcony or deck.
  • Attached vs. Detached:The zoning code standards for attached versus detached ADUs differ as follows:
    • Detached buildings. To be considered a detached building, the following conditions must be met:
      • Zoning code - The ADU must be at least 60 feet from the front property line, or the ADU must be at least six feet behind the house, attached house or manufactured home.
      • Building code - In order to meet fire life safety requirements, all detached ADUs must be separated from the primary building by at least six feet.
    • Attached buildings. To be considered attached, living space of both the primary struture and ADU must abut (be on opposite sides of the same wall). Incidental and accessory features such as trellises, decks, patios, breezeways, garages or tool sheds will not be considered as establishing an attached structure.
  • Parking: Additional parking is not required for an ADU. However, if parking is required for the existing dwelling unit, that parking must either be retained, or if eliminated in the creation of the ADU, replaced.
  • Other Uses: A Type A home occupation can be on the same site as an ADU. Type B home occupations are prohibited from being located on the same site as an ADU. If a Type B home occupation is already located in a house, then a subsequent application for creating an ADU will be denied unless the home occupation is removed.
  • Design Review: Design review is required for development of ADUs in a ‘d’ overlay zone, when external modifications to the site or existing house are made. These ADUs must meet the Community Design Standards, where allowed, or, alternatively, be approved through the design review process.

(2) Construction Standards

The ADU Program Guide outlines the construction standards, such as headroom, room size, unit separation, and utilities for ADUs.

When converting an existing unfinished space to an ADU, some features, such as ceiling heights, windows, stairs and insulation, may not meet current building code requirements for finished space. These conditions could make it expensive, difficult or even impossible for you to change your attic, basement or garage into living space.

(3) Water

Regulations require that each separate structure have a separate water service. For more information call the Bureau of Water Works at (503) 823-7368.

(4) Sanitary and Storm Sewers

The ADU can either connect to the existing sanitary sewer system or have a separate connection. If a separate connection is made, a sanitation permit is needed for the installation.

(5) Decommissioning a Septic Tank or Cesspool

When building an ADU, the septic tank/cesspool must be located (if one exists).

  • Visit the records staff at the Development Services Center to locate any old plumbing records showing a septic system/cesspool
  • If the septic system/cesspool is decommissioned, and join ten feet of the new structure, you must sign a disclaimer for on-site-sewage disposal system form
  • If the septic system/cesspool is within ten feet of the new structure, it will need to be decommissioned

Call Environmental Soils staff at (503) 823-6892 for more information on decommissioning septic tanks and cesspools.

(6) If any ground disturbance, erosion control review and inspection may be required.