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Accessory dwelling units: what further research should be done?

[This is part 12 of a 13-part series on ADU research and policy.  Read the series intro and table of contents.]

Most term papers, dissertations, and even published journal articles hit rock bottom about 90% of the way through.  That’s the murky, dark and tangled section where the author is obligated, either by literary form or their instructor’s/editor’s insistence, to report their doubts about the findings they have related, and suggest opportunities for further investigation.

photo of murky forest

Murky forest photo by Eddie Hammond (Creative Commons)

Too often this section reads like the author has completely thrown up their hands and confessed to sudden incompetence.  “Well, we studied subject A, using method B, but the truth is one could have doubts D, E, F, G, H, I, and J.  Until D, E, F, G, H, I and J are investigated, we really won’t know anything.”  This is an uninspiring lead-in to the next topic — “And by the way we need more funding!”

I hate this routine, because it suggests the author doesn’t really have a sense of how their work has truly contributed to the field, and what questions their work has truly left unanswered.  Really, what they should say is this.  “I studied subject A using method B, and now I know something more: C.  Yes, C!  C isn’t much, but it’s something that adds to the field.  The field would be even richer if we could answer D, E, F, G, H, I, and J.”

I’m not going to throw up my hands and act like the numbers mean nothing.  Knowledge about ADUs has improved a lot in the past few years.  Here I’ll state how it’s improved, and what I believe should be studied next.

Back in 2011 there was little credible available information about the most basic aspects of real ADUs in the United States.  There had been a few pieces of research in the 80’s and 90’s but those seemed out of date.  It wasn’t clear how many ADUs existed, who lived in them, how much they cost, etc.   This lack of basic data made evaluating the many claims in civic debates about ADUs (e.g. “they will cause parking problems” or “they will create affordable housing”) an exercise in rhetoric, not evidence.

Since then several original studies have been published that contain real data about existing ADUs, not just compendiums of policy or claims about what ADUs could do. These were:

If I have missed any published source with actual credible data in it, please let me know.  I should also note that for better or worse, I am the “Brown” in the list above. 🙂

I’ll go over a list of the major findings in the final post in this series, but here I want to draw the most promising and dramatic results out, because these are areas where future research could productively focus.

Research topic: exactly how green and car-light are real ADUs, compared to other forms of housing?

The results so far suggest that ADUs are extremely promising in environmental terms.  Population is growing, and people will be housed somewhere.   The biggest influence on a dwellings’ long-term environmental footprint is its size.  ADUs are not just small, but well-suited to the 1- and 2-person households that are common today.

Here’s the estimate I came up with:  the typical Portland ADU will have half the environmental footprint of the typical new single family home — and yet, in all probability house the same 1- or 2-person household.

graph shows that ADUs have about half the climate change impact of a standard size home

Estimated climate change impact of a median sized ADU compared to standard SFR. The estimate for the ADU comes from a regression on the 4 Oregon DEQ (2010) data points (in circles).

Now this result is a projection based on extensive modelling, so there are certainly reasons to be a bit skeptical.  That’s why I believe the next step in studying the environmental impact of ADUs should be a field study.  In particular, the study should compare the actual resources consumed by ADUs to other forms of housing used by 1- and 2- person households.  A field study — for example, based on construction records, or based on utility bills (for ADUs that are metered separately) — could cut through all the messiness and all the variables people want to throw up their hands about, for example, differing ages of buildings, architectural forms, etc., and show directly how these dwellings impact the environment.

It seems quite likely that ADUs will beat out single-family residences, but how will they compare to apartments in “big multifamily” developments?  This is a key question.  For many communities, there is no question that densification is inevitable.  But those communities have a choice about the form it will take, and ADUs may be a palatable option compared to huge apartment blocks.   If ADUs can provide the same kind of environmental benefits as “big multifamily,” but provide them in a more neighborhood friendly way, they may be the way to go.

Besides a dwelling’s size, another major factor in a dwelling’s environmental impact is its compatibility with transit and other alternative transportation.  ADUs also seem to have major promise here, as my graph showed:

bar graph showing average vehicles per household type

ADUs (in Portland, anyway) seem to be nearly as car-light as those “transit-oriented developments” (TODs) that have been explicitly built to take advantage of transit.   It should be acknowledged that this is a finding based on a rather limited data set, but it makes sense when you consider the kind of relationships that tend to exist between ADU owners and tenants (see some case studies).  I speculate that these combined households might be sharing transportation somehow.  In any case, the trend is compelling enough to deserve further attention — in other cities, and in a broader range of dwelling types.

I believe that a study of vehicle ownership and transportation use among various housing types, including ADUs, could be very revealing.  Though I can’t describe a whole study design here, it might be possible to compare an analysis of the Census’ American Housing Survey microdata with original data about ADU occupants.  Or perhaps totally new and focused surveys could do a more credible job of comparing vehicles and transport among housing types.  Again, I think it’s vital to compare whatever is happening in ADUs to the alternatives to ADUs in a densifying city — for example, to standard single-family residences, and to “big multifamily.”

Research topic: exactly how do ADUs influence housing affordability, for BOTH homeowner-developers and tenants?

Gentrification and affordable housing are topics that are tormenting cities around the country.  As neighborhoods get denser and housing prices rise, how is it possible to keep a vibrant variety of residents, with different ages, incomes, and cultural backgrounds?

It seems fairly clear that the institutional kind of affordable housing, though entirely necessary, will not be sufficient to meet this need.  What more can be done?

Though the discussion is deeply confused by vague and conflicting definitions of affordability,  it is clear that ADUs have a lot of potential as a flexible, free-market source of affordable housing.   They offer small apartment-sized units in mostly house-sized areas, they are very cheap to build compared to institutional affordable housing, they aid homeowners in making their own house payments, and about 20% of them are voluntarily rented for far less than market rate.

graph of ADU rents in Portland, Oregon circa 2013

Before this helpful phenomenon can be productively used by policy-makers, the extremely unusual relationship of “developer” and “tenant” must be understood.  ADUs are completely unlike other kinds of housing in that they are usually developed by regular homeonwers, not by real estate professionals. Even though these homeowner-developers are most often motivated by financial need, they do not always act to maximize investment return, and that’s where the benefit to the community may be created.   For months or years at a time they may provide low-cost housing to a family member or friend, one who might otherwise need government assistance.

I believe this phenomenon needs to be understood in detail before regulators try to harness it.  Currently, it seems that choice and flexibility of use are part of the motivation when owners create ADUs in the first place.  So I speculate it may not be productive for local governments to require ADUs to be affordable — because without flexibility, ADUs may not be created.

A worthy research project would investigate the affordability of housing for both the homeowner-developers and the tenants of properties with ADUs.   That means at a minimum, the researcher would need to get rents or mortgage payments, utility costs, and income figures for BOTH parties, as well as the construction/purchase cost of the ADU.  Then those figures could be analyzed to find if the ADU improves affordability for both parties, only one of them, or neither.

Such a study might also work in demographic examinations of ADU owners and tenants, for example to further investigate the benefits of ADUs for older citizens.  It could discover the scenarios that help create free-market, unregulated affordable housing, which might help policy makers encourage it.

Research topic: how do permitted and unpermitted ADUs differ?

There is so little actual data about real ADUs that anyone studying them must look at any information that’s out there.  But I suspect, based on personal experience, and a comparison of Wegmann’s East Bay study to the results for Portland, Oregon,  that permitted and unpermitted ADUs may have some notable differences.  In particular I suspect that, on average, unpermitted units are not as well-built or full-featured as permitted ones, and that these disadvantages are represented in their rental prices.   Accordingly, unpermitted ADUs might not provide a completely accurate representation of the effects of permitted ADUs.

So — without being judgmental about unpermitted ADUs 🙂  — a worthy research project would investigate the differences between a collection of permitted and unpermitted ADUs in the same geographic area.   Differences could be physical (qualities of the building), demographic (qualities of the owners and/or tenants), or economic (rents, incomes, and mortgage payments).

Research topic: how many are there?

Finally, I think it would be worth studying how many ADUs — both permitted and unpermitted — exist in major metropolitan areas, and comparing those numbers to the number of available properties.

I think this simple set of numbers could be very valuable. I predict that:

  • In most areas, there are likely to be very few ADUs of any kind.  For example, currently the ADU “mecca” of Portland has about 800-1000 permitted ADUs out of >140,000 properties where they would be allowed.   Even if you add in unpermitted ADUs, the numbers are likely to represent <1% of available sites.
  • Some areas, for example around Boston or San Francisco, will have much higher rates of ADU presence, perhaps 2% to 10% of available sites — and nearly all of those units will be illegal.  This will demonstrate that when economic motivations are strong enough, even tight regulations will not stop ADUs from being built.

Such a simple data set would put divisive ADU debates in proper perspective.  If it is true that ADUs tend to be uncommon and in any case are developed slowly, most diehard opponents of ADUs don’t have much to worry about.   And if it is true that economics can drive ADU creation more powerfully than rules can prohibit them, that suggests that local governments might want to find ways to create the kind of ADUs they can accept — rather than exerting no influence over ones that will pop up in defiance of the rules.

To wind up

These are some of the significant findings and discussion points based on my study of the field so far.  They aren’t written in stone — it’s possible new evidence could demonstrate that I’m completely wrong.

But at this point, there is no reason for me to let my presentation get bogged down in that murky, dark and tangled apology that is so traditional in academic writing.  We’ve learned a lot about the reality of ADUs in the past few years.   Sure, none of that research was perfect, and there’s a lot more to learn.  But the kind of results in the graphs above show that ADUs have remarkable potential, and it would be well worth the effort (and budget! 🙂 ) to discover more.

next week: the big conclusion: are ADUs as great, or as horrible, as people have imagined?

About Martin John Brown

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

4 comments on “Accessory dwelling units: what further research should be done?

  1. Pingback: Research and policy about accessory dwelling units: introducing a 13-part series | Accessory Dwellings

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

  3. Brian
    March 10, 2015

    We’re seriously planning an ADU for our home, I think everyone who is thinking about building an ADU in the Portland area is coming to this site. This could be a perfect place to do a survey of those that are planning to build or that have built already, allowing you to extract owners experiences, and add to our collective knowledge base!

    • Martin John Brown
      March 10, 2015

      Hi Brian, this series of posts actually does what you’re talking about — it interprets the results of a survey of Oregon ADU owners with the help of case studies from this site. Enjoy!

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This entry was posted on September 3, 2014 by in Policy & Trends and tagged , , , , .
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