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Accessory Dwelling Units (ADUs)

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Source: Accessory Dwelling Units (ADUs) ↗ Last modified: Sun, 19 Apr 2026 19:57:46 GMT

An ADU is a dwelling built on a lot that already has a house, attached house, duplex, or manufactured home. The ADU must be smaller than the main dwelling. You can create an ADU by:

You'll need building and trade permits for this project

We require a building permit ↗ if you want to convert an attic, basement, or garage to a living or habitable space. You'll also need a building permit to build a new ADU ↗. 

For convenience, the City of Portland offers pre-approved building plans for detached ADUs. If you want to build one of these, you should submit those plans with your building permit application. See Step 2 below for more information about pre-approved plans. 

Your ADU project will also need trade (electrical ↗, plumbing ↗, and mechanical ↗) permits. You can apply for trade permits at the same time as your building permit, or you can do that later. The advantage of applying for both kinds of permits at the same time is that you'll have one permit number. But you can apply and pay for trade permits later in the process.

Check with your county assessor on likely property tax impacts

Adding an accessory dwelling unit will likely increase the value of your home. If the value of your home increases, your property taxes will too. The county assessor, not the City of Portland, determines your property taxes.

Before starting your project, contact your county assessor. They can help you learn how your taxes may change. 

Evaluate existing space for ADU conversion

Constructing an ADU within your existing home may be an option. But you should be aware that some areas in your home might not meet requirements for a living space.

Common issues can include:

  • Ceiling height
  • Emergency exit windows (also known as "egress windows")
  • Insulation and ventilation
  • Stairs ↗

These issues could make it expensive, difficult, or even impossible for you to convert a space into an ADU. Use the information in Step 1 below to start researching your property.

Visit ADU conversions ↗ for full details on converting an existing space, like an attic, basement, or garage, into an ADU.