Accessory Dwellings

A one-stop source about accessory dwelling units, multigenerational homes, laneway houses, ADUs, granny flats, in-law units…

Research and policy about accessory dwelling units: introducing a 13-part series

For years civic debate about ADUs has been like certain questions among sports aficionados.  Who was the better baseball player, Babe Ruth or Willie Mays?  The discussion goes round in circles, because there simply is not much evidence to be had.  Ruth and Mays played in different eras, and certainly never against each other.  That means that besides a few statistics, much of the “evidence” that can be introduced into the debate is tangential or speculation, a mere riff on the speaker’s foregone conclusion.

With ADUs, this combo of pictures is a good preview of the debate.

contrasting photos of anti-ADU protesters and happy ADU homeowner-developers

photos by Steve Lewis of the Durango Herald (left) and Laure Joliet of the New York Times (right)

On one side are the advocates, to whom ADUs seem like a progressive no-brainer — little houses in backyards, or daylight-basement apartments, that they say will provide some small, environmentally sensible housing, provide affordable rentals, help homeowners pay their expensive mortgages, and let seniors age in place, all without changing the basic nature of single-family-residence neighborhoods that Americans seem to so much.

For detractors, ADUs are a lurking danger — a harbinger of crowding and decay.  Letting ADUs into existing neighborhoods, they say, will threaten parking spaces, property values, and neighborhood quality of lifeADUs will change the nature of neighborhoods, the opponents insist.

Based on anecdotes, my personal guess has always been that the advocates were more likely to be right, at least on most points.  But as a professional researcher I have kept a wait-and-see attitude.  Which of these hopes and fears can be supported by evidence?

Sadly, there has been very little evidence to work with.  For all the ink spent on the issue in the past 15 years, shockingly little has been published in that time that contains actual data about actual ADUs, the people who own them, and the people who live in them.  In 2001, there was a small study of the owners of attached ADUs in Seattle.  In 2012, I co-authored a study of methods to appraise the value of ADUs.  In 2012, there was a study of accessory dwelling units in the San Francisco Bay area.  In 2013 there was a survey of Oregon ADU owners, but the results were presented without any interpretation.  That’s pretty much it!

That’s why I’m  glad to announce a new collection of evidence about ADUs is now available.  With a contract from the Oregon Department of Environmental Quality‘s Green Building initiative, and with supervision from the DEQ’s Jordan Palmeri, I re-examined the results of the 2013 survey of Oregon ADU owners.  In particular I compared Portland ADUs, their owners, and their tenants to carefully chosen statistics about the wider community of Portland housing and residents.  My aim was to test eight common claims and questions about ADUs.  I don’t think my findings will be the end of the story by any means, but I do hope they are the beginning of a more objective discussion of the issues.

You can download and read the whole report here, or here [alternate link] , if you’re up for 47 pages of references and statistical tests.  But if not, or you want to talk about the issues in more detail, please follow my series of posts for the next 13 (or so) weeks.   Once a week, I’ll be investigating a different hope, fear, or open question about ADUs, and trying to lay out the evidence fairly.  Here’s the schedule:

I hope you’ll follow along!

– Martin

ps.  Ruth was better, obviously.

About Martin John Brown

Martin John Brown is a researcher and consultant on environment and housing. Find out more at

19 comments on “Research and policy about accessory dwelling units: introducing a 13-part series

  1. Sue Thering
    June 4, 2014

    Martin John Brown you are a treasure! And Oregon Department of Environmental Quality is a national model of thoughtful, evidence-based decision making. And is a great resource. Thank you all!

  2. Kathryn Goldman
    June 5, 2014

    You’re kidding, right? Mays, all the way.

    Looking forward to this series of posts. We’re planning to build an ADU in Baltimore City to retire in place. Gathering information to figure it out and deal with Code Enforcement Dept.

    • Tiffany Nicolette
      July 31, 2017

      I live in Baltimore City and am thinking of exploring this option for my father-in-law. I know this is an old post, but I’m wondering if you had success with this.

  3. Pingback: Why ADUs are hard to study, and confuse nearly everyone | Accessory Dwellings

  4. Pingback: Moment: Accessory Dwelling Units | HOMEmakers of Oregon LLC

  5. Pingback: Do ADUs actually provide housing? | Accessory Dwellings

  6. Pingback: Do ADUs actually provide housing? | Accessory Dwellings

  7. Pingback: Do ADUs cause neighborhood parking problems? | Accessory Dwellings

  8. Pingback: Do accessory dwelling units serve older persons? (Or, who lives in, and owns, ADUs?) | Accessory Dwellings

  9. Pingback: How do ADUs contribute to the local economy? (Or, will building an ADU raise my property taxes?) | Accessory Dwellings

  10. Pingback: Do ADUs provide affordable housing? | Accessory Dwellings

  11. Pingback: Are ADUs green housing? | Accessory Dwellings

  12. Pingback: How do ADUs affect property values? | Accessory Dwellings

  13. Pingback: How much do ADUs cost to build? | Accessory Dwellings

  14. Pingback: What are the barriers to ADU development? | Accessory Dwellings

  15. Pingback: Accessory dwelling units: what further research needs to be done? | Accessory Dwellings

  16. Pingback: Summing up ADU research: are accessory dwelling units as great, or as horrible, as people say? | Accessory Dwellings

  17. Pingback: Do lax “granny flat” regulations encourage absentee landlords? Some new evidence from Portland. | Accessory Dwellings

  18. Pingback: A win-win-win solution for affordable housing - Outer Banks Beacon

Comments are closed.


This entry was posted on June 4, 2014 by in Policy & Trends and tagged , , , , .
%d bloggers like this: