Basement ADUs: 2020 Portland Guide

basement adus portland oregon

For many Portland homeowners, having an Accessory Dwelling Unit might be the ticket to affordability, renting out a second unit on your property. Or, an ADU might allow you to take care of an aging relative, or simply create housing out of what was unused space. 

Most people think of a backyard cottage when they think of an ADU, but in fact, many Portland homeowners have legal ADU potential right under their feet: in the basement! Now that Portland has updated its zoning rules to allow for ADUs on most city lots, 2020 may be your year to build a basement ADU.

But before you start picking out wall colors and furnishings, there are four essential questions you must ask yourself (or your tape measure) to know whether your basement can accommodate a legal, permitted ADU.

Portland Basement ADUs: Four Questions to Ask (before you get started)

  1. Will the space meet basic building codes? Portland requires that legal ADUs have a ceiling height of at least 6’8” and their own exterior entrance. In addition, any bedroom in the ADU must have an egress window (at least 20” x 27”). If your basement can’t meet these basic requirements, or if you aren’t willing to pay a lot more to reconfigure the space, you can still finish it as usable space — you just can’t call it an Accessory Dwelling Unit. 
  2. Is your home in a Flood Hazard Area? Being in this area may prohibit you from building a basement ADU, but call Portland Development Services before you shred your plans. If you don’t know whether your home is in a Flood Hazard Area, go to, enter your address, and look for the Flood Hazard Area information in the sidebar that comes up. Also make sure to waterproof, as needed, before getting started. No one needs an unexpected flood to ruin all of their hard work.
  3. Will building the ADU still allow you to access your furnace and electrical panel? Portland code requires that there be direct access from the main house to all utilities. For most homes, the furnace, electrical panel and hot water heater are going to be in the basement. Your basement’s configuration might make it difficult to both access utilities and create a separate living unit. But if this is your only deal-breaker, there are always workarounds, like adding a hallway. Meanwhile, ADUs must have their own heat source and electrical panel. Be careful when finishing a basement! Make sure you still have access to plumbing shut-offs, electrical box junctions and more to avoid problems later.
  4. Do your personal preferences really line up with having downstairs neighbors? Although the ADU’s tenants will be able to use their own exterior door, consider their entire path of exit/entry, including any downstairs windows they’ll need to walk by to get to the street. Are you ready to give up a little privacy? Most people who have basement tenants also find that it’s noisier than expected (though most of the noise is experienced by those downstairs!). There’s a certain amount of soundproofing that can be done, but do know yourself and whether you’ll be able to live with the changes to your living situation. 

Two Essential Elements for ADU Safety

If your home cleared the above requirements – great! Chances are good that you can create the basement ADU you’ve dreamed of. However, there are two important safety considerations to work into your plans as well. 

  1. A seismic retrofit. There’s no getting around it, Portland is due for a couple of big earthquakes. Is your home ready to shake? Seismic retrofitting (bolting the home to its foundation) is recommended for all Portland homes built before 1990, and if you’re going to finish your basement, doing the retrofit now is a very smart idea. In basement remodels such as ADU conversions, the studs and rim joists are often exposed, making it easy and cost-effective to do seismic enhancements. Once those studs are covered up and the unit is occupied, however, it will be very difficult.
  2. A radon check. This odorless, but carcinogenic gas is found underneath homes across Portland, so if you’ve never had a test done, now’s the time to do it. If there are high levels of radon in the basement, a mitigation strategy should be put into place before you move on with the project. In addition to checking for radon before you get started, you should  check again after any basement remodels. Changing the structure and layout of your basement can change the way radon moves from the earth and through your home.

Portland Basement ADU Resources

Planning a basement remodel that meets all the requirements for an Accessory Dwelling Unit in Portland can be intimidating — but there are plenty of Portland-based resources to help through it. Here are some of our real estate agents’ favorites:

  • City of Portland Building and Development Services. Portland’s official guide to ADU construction has everything you need to stay legal and get the permits you need. And, Systems Development charges are waived for Portland ADUs that aren’t being used for short-term rentals!
  • This comprehensive site, run by Portland-based ADU advocates, gives you just about everything you need to know about building an ADU. There are pictures and stories of several basement ADU conversions, and there’s even a page for ADU regulations by city, so you can check the rules for building ADUs in Beaverton, Hillsboro, Gresham, or wherever you are in the Portland area. You can also check this website for notices about ADU workshops coming up in the Portland area, which cover topics like ADU design and navigating the permitting process.
  • Golden Basement. This Portland-based consultant has built several ADUs and helps homeowners through the process of planning, permitting and building their own ADU. If you aren’t sure where to start and want to get an accurate idea of how much your project will cost, a consultation could be well worth the fee. They also have a helpful blog.
  • Your Portland real estate agent is your best place to start if you’re looking for a home that can accommodate a basement ADU. As you probably have learned by now, not every basement can legally be converted into an extra unit. Get an extra set of eyes to help you look for that perfect property — sometimes it just takes a person who is able to see the potential.

The spring buying season is upon us so don’t miss your opportunity to work with a Top 1% Portland Buyers’ Agent, and find the home of your dreams for your future ADU project. Contact us today!

February 28, 2020

Stephen FitzMaurice

Stephen FitzMaurice, Realtor is a top 5% real estate agent in the U.S. A Principal Broker in Oregon, Managing Broker in Washington, he has been licensed since 2003 for residential real estate sales. Call his team in Oregon at 503-714-1111 or in Washington at 360-345-3833.

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