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.
- Completed building permit application form
- Completed NSFR Intake Packet if proposing a Detached ADU
- Completed residential system development charge form
- Site and building construction plans:
For brand new construction, see the submittal standards and drawing criteria in the NSFR Intake Packet (see above).
For altering an existing structure to add an ADU, see the submittal standards and drawing criteria in Adding a Dwelling Unit Submittal Requirements.
- Erosion control plan (may be a part of the site plan) or completed simple site erosion control form, if the project will result in ground disturbing activity. Note: Erosion control measures shall be installed, inspected and approved even when a simple site erosion control form has been accepted.
- Mitigation form and/or a stormwater plan if your project will add more than 500 square feet of impervious area
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.
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.
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 https://multco.us/assessment-taxation/news/are-you-considering-building-accessory-dwelling-unit for more information.