1900 SW 4th Ave, Portland, OR 97201
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 https://multco.us/assessment-taxation/news/are-you-considering-building-accessory-dwelling-unit for more information.
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
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.
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. Without these signed applications on file, you will have to apply for trade permits separately from the building permit. Sub-contractors you hire must be registered with the State of Oregon and carry a current CCB license.
(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.
(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.
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).
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 https://multco.us/assessment-taxation/news/are-you-considering-building-accessory-dwelling-unit for more information.
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.
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 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.
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.