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Development Services

From Concept to Construction

Phone: 503-823-7300


1900 SW 4th Ave, Portland, OR 97201

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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.

ADUs and Property Taxes.  Construction of an ADU could result in a significant increase in property taxes under Oregon tax law. So before you start an ADU project, it's important to contact the Multnomah County assessor's customer service line at 503-988-3326 or visit their website at for more information.

Getting Started

An accessory dwelling unit (ADU) is a second dwelling unit created on a lot with a house, attached house or manufactured home. The second unit is created auxiliary to, and is smaller than, the main dwelling. ADUs can be created in a variety of ways, including conversion of a portion of an existing house, addition to an existing house, conversion of an existing garage or the construction of an entirely new building. 

Methods to create an ADU

  • Chapter 33.205 of the City of Portland Zoning Code provides the standards for accessory dwelling units in all Residential (R) zones. ADUs can be created in a house (detached single family dwelling), an attached house or a manufactured home any of the following ways:

    a. Converting existing living area
    b. Finishing an existing basement or attic
    c. Building a new structure
    d. Making an addition to an existing structure

    Where attached housing was created under the infill provisions of the ‘a’ overlay zone (section 33.405.060), or as duplexes and attached houses on corners (section 33.110.240 E), ADUs are prohibited. Attics and basements with limited ceiling heights may not qualify as space that can be converted.
  • Some existing attached or detached garages can be converted into an ADU in all R zones. ADUs can also be created by construction of a new accessory structure.

Visit the Planning and Zoning staff at the Development Services Center to determine the specific limits applicable to your property.

A building permit is required to build an accessory dwelling unit. Depending on the scope of work, your project will most likely also require electrical, plumbing and/or mechanical permits. 

For new construction, your permit will be reviewed under provisions of  the  current Oregon Residential Specialty Code. View more information on building codes.

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.

The Bureau of Development Services sells a "Residential Combination" permit package. This allows you to call for all inspections using just the Building Permit (RS) IVR number and pay for all the permits at one time. You will need to submit the trade (mechanical, electrical, and/or plumbing) application(s) signed either by you, if you are doing these portions of the work yourself, or by your licensed subcontractors. If you do not have your completed subcontractor forms when you are applying for your residential permit (RS) you will have to apply for trade permits separately from the building permit. Sub-contractors that you hire must be registered with the State of Oregon and carry the correct licenses in association with their trade.

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: Additional requirements apply to detached ADUs, as follows:
    • 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.
  • 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 or two or more houses or buildings under one ownership and on the same lot or parcel of land may be supplied through a single service meter. The property owner must sign a “Separate Service Agreement” which will be recorded on the property which stipulates if the property is divided a separate water service shall be obtained for each parcel. 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.

(7) ADUs and Property Taxes

Construction of an ADU could result in a significant increase in property taxes under Oregon tax law. So before you start an ADU project, it's important to contact the Multnomah County assessor's customer service line at 503-988-3326 or visit their website at for more information.

Plan Review Process

Plan reviewers from the Bureau of Development Services representing fire life safety and structural disciplines, as well as staff from the Parks Bureau, Office of Transportation, Bureau of Environmental Services, and Water Bureau, will all participate in the review of your plans.

The bureaus/review groups that will check a typical conversion project include:
  • Planning and Zoning
  • Life Safety Review
  • Structural Review
  • Addressing
  • Site Development Review
  • Residential Subsurface Site Evaluation - Site Development
  • Bureau of Environmental ServicesReview
  • Portland Department of Transportation Review
  • Bureau of Water Works Review
  • Urban Forestry Review
See the Permit Processing Flowchart

A checksheet is sent to the applicant when a reviewer needs additional information or a correction has to be made to the plans. When you have gathered the additional information or made the corrections, bring the plans/information to the Development Services Center. Reviewers will be notified that a checksheet response has been received and the review will continue. More information on checksheets.

Status Reports
Status reports are available after your project has been assigned to reviewers. Status reports show all reviews and include the reviewer’s name and phone number. To obtain a status report, call (503) 823-7000 and select option four to have a status report faxed to you or call (503) 823-7357 to have one mailed to you.

Permit Issued
When the last technical review is approved, your permit will pre-issued. You will be contacted when your permit is ready, and of your final fee total. Fees may be paid by cash, check, Visa or Mastercard.

If your permit is ready for issuance, you may pick up your permit between 8:00 am - 3:00 pm, Tuesday-Friday at Permitting Services, located on the 2nd floor of1900 SW 4th Avenue,Portland,OR. For more information call (503) 823-7357.


The yellow inspection card lists all the inspections you will likely need during your construction project, and what work needs to be done first.

Once your building permit is issued, erosion control measures must be installed, prior to beginning any further ground disturbing activities.

Applications & Handouts