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Why Are Permitted ADUs So Rare?

From Washington DC to Vancouver BC to Boston, MA, there are more and more examples in the media of individuals attempting to build and live in smaller dwellings and in more flexible living arrangements. Passive income, multi-generational housing, and other cooperative housing models are the common homeowner motivations for developing these kinds of housing options.

Where ADUs are permitted, they provide a viable mechanism to allow for this kind of smaller, flexible housing arrangement.

A homeowner building an ADU for rental income. From ‘Bend homeowners add rentals’

While some cities promote ADUs, most don’t. And many cities may claim to allow ADUs, but only under prohibitive zoning preconditions. Here are the common preconditions:

  • Municipalities may require construction of an additional parking space on the lot if you build an ADU; this is an impractical or impossible requirement for many urban lots.
  • Others will allow ADUs, but will require homeowner occupancy in one of the two units.
  • Other cities may allow ADU construction only if neighbors approve of the concept, instead of having a predictable “by-right” path of development.

And once a homeowner overcomes the labyrinth of their municipal zoning codes, they must address building codes, which are more onerous in smaller, secondary dwellings than they are for larger primary residences. These regulatory factors all slow the rate of permitted ADU construction that would otherwise exist in their absence.

Meanwhile, there are many examples of non-permitted, smaller, secondary housing options in cities all over the country. This ‘gray market’ is a testament to the demand for these smaller, flexible housing options. Unfortunately, the non-permitted ADU housing stock can also stain communities’ notions of whether ADUs are ‘good for their neighborhood’. Their idea of an ADU may be run down, unmaintained, sub-par non-permitted housing forms that they’ve seen with their own eyes. They haven’t actually seen permitted ADUs before because, well, there aren’t any to see.

This false perception of ADUs as poor, run-down construction is particularly ironic because permitted ADUs are typically constructed to a higher standard since they must adhere to newer and stricter building codes than their respective (older) main houses.

Over the last six months, Sightline’s Alan Durning has been authoring a wonderful series about the inanity of various forms of overregulated housing forms. Three of the last posts deal explicitly with ADUs: the hidden density that they can provide, how relatively rare permitted ADUs are, and the planning and zoning regulations that prevent more rapid ADU construction. We encourage readers to read these articles:

These articles expand upon the onerous zoning regulations that often apply to ADUs and help explain why permitted ADUs are so rare.

The disparity of U.S. household types and home construction types.
More at

Current housing options aren’t meeting the demgraphic housing demand for smaller detached units.

We know there is an unsatiated demand for smaller dwellings, a demand that correlates with our changing home demographics as shown above. I look forward to seeing how this gray market will turn into a more viable market as more and more forward-thinking cities start to adopt permitted ADU construction as an explicit policy goal.

About Kol Peterson

Kol is an ADU consultant, advocate and author of Backdoor Revolution: The Definitive Guide to ADU Development. Read more here: and learn about building your own at Email at

5 comments on “Why Are Permitted ADUs So Rare?

  1. Olivia Schelly
    May 24, 2013

    I am looking to build a ADU in NE Portland and I am having a hard time finding out how close to the property line you can build. I found the set back guidelines, but nothing on side or back property lines. I was wondering if you have any insight on this. Thanks.

    • kolpeterson
      May 29, 2013

      Olivia, it’s best to ask BDS directly, but generally speaking, you can’t build any structures within 5ft of your property lines. There’s exceptions to this case for conversions of some pre-existing structures that are within the 5ft setback and there’s also an exception if the ADU will be built off an alley, in which case you can build right up to the alley lot line.

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This entry was posted on March 30, 2013 by in News, Policy & Trends and tagged , , .
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