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Are ADUs green housing?

[This is part 6 of a 13-part series about accessory dwelling unit research and policy.  For the series intro and table of contents see here.]

Environmental impact rarely comes up in civic debates about accessory dwelling units.

Rather, both advocates and detractors of ADUs seem focused on the social milieu they hope (or fear) ADUs will engender — for example the hope that more housing options will become available, or the fear that local property values will go down.   “Environment” is considered mostly in the architectural and neighborhood sense.

But what about environment in the sense of carbon footprint, resource conservation, and so on?

Photo by Kevin Dooley (Creative Commons)

Can those windmills make up for all that highway?  Photo by Kevin Dooley (Creative Commons)

In my opinion, ADU advocates should be talking about this a lot more, because environmental impact is an area where ADUs seem to have a lot of potential — at least in comparison to the standard American model of housing, the “single family residence (“SFR” in planner-talk).

Density and transportation

As infill that increases density, ADUs probably have an advantage over the typical subdivision model of development — after all, a lot of the infrastructure is already there for the ADUs to connect to.  Studies of density and environmental impact support this idea, for example Ewing & Rong’s finding that residential energy use increases with sprawl, & Brownstone & Golob’s finding that vehicle and fuel use per household decreases as density increases.  In Portland, we find that ADU households have significantly fewer cars than other kinds of households.  (I’ll report on this more in a future post, but if you want you can read the results in my research paper.)

The housing mismatch

Perhaps more significantly, ADUs seem to respond to the growing mismatch between American housing and American households. There are a lot of small households knocking around in a lot of big houses, and that has an effect on the environment.


  • Average US household size has declined, from 3.7 persons/household in 1940 to 2.6 in 2011.
  • More than 60% of American households are just 1 or 2 people.  They might be empty nesters, younger childless couples, or people on their own by choice or circumstance.  Regardless of the reasons, these people don’t need huge dwellings.
  • Meanwhile the median size of the country’s most popular form of housing, the single family residence, has been increasing in new construction, from around 1000 square feet in 1950 to 2306 sf in 2012.
  • Accordingly, median square footage per person in new construction has been increasing, from 676 sf per person in 1993 to 850 in 2011.

Size the biggest factor in environment impact

As houses have grown, evidence has accumulated that size is likely the single largest factor in the environmental effect of a dwelling, in terms of both energy and materials. The Oregon DEQ has conducted detailed modeling of the long-term effects of various green housing technologies, comparing about 25 green building practices such as better insulation and windows, increased air tightness, advanced framing, and reducing home size. Their study found that, of all the practices studied, reducing home size was the most beneficial practice for reducing both material and energy related environmental impacts. Comparing a “medium” house of 2262 square feet (which is very close to the current median size in new construction) to an “extra small” house of 1149 sf, it found the extra small house reduced numerous measurements of environmental impact by 20-40%, while the climate change impact was reduced by 36%.

Though this kind of finding comes from a model, not a field study, it’s been influential.  Size has (properly) been incorporated into environmental building certifications such as LEED for Homes (which adjusts scoring to account for size).

How ADUs fit in to these trends

Since ADUs tend to be small, both by convention and regulation, they look like they will be clear winners.  The median size of a detached ADU in Portland is 700 square feet — considerably smaller than the “extra small” house in the Oregon DEQ study.   It should have a small environmental impact.  Here’s a graph where I show how such an ADU would fit into the size/climate change impact trend.

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 “medium-sized” (2262 sf) single family residence. The estimate for the ADU comes from a regression on the 4 data points marked with circles, drawn from Oregon DEQ (2010) Figure 45.

The DEQ report’s relationship between climate change impact and house size is remarkably linear, as my regression line shows.  I have extrapolated the trend to estimate that a detached ADU of 700 square feet will have a long-term climate change impact that is 50% less than a “standard-sized” house of 2262 square feet.

This is a dramatic result.   Of course it is an extrapolation, and statistical weenies will object, rightly, that extrapolations should be viewed with caution. But the DEQ model gives a very clear trend, and the extrapolated point is fairly close to an observed data point.  Assuming the DEQ model is at least somewhat accurate, it is fair to say there is a strong effect here.

Incorporating household size

A more substantial objection to the idea that ADUs are superior for the environment might be based on household size.  Of course ADUs are smaller, one might say, but they house fewer people as well, so on balance the ADU advantage in climate change impact could be no advantage at all.

Looking at Portland, I find that detached ADUs have a mean household size of 1.39 persons, while the most comparable SFRs (i.e. newly constructed rental SFRs) have a mean household size of 2.26 persons, in about 1600 square feet of living space.  From these kind of numbers, we can calculate a more standardized metric, living area per person.

graph of living area per person in various housing forms, showing that ADUs are usually lower than SFRs
The results depend on exactly what groups are compared, but ADUs win out most of the time.  The living area per person metric is significantly smaller in Portland ADUs than in:

  • newly constructed rented SFRs (which tend to be large);
  • all SFRs: and
  • the “primary” SFRs (i.e. the ones which were associated with the ADUs).

One group of SFRs bucked this trend.  The group limited to rented SFRs of any age showed a smaller living area per person in Portland than ADUs.  This is probably due to the large number of 50-100 year old houses in Portland (which tend to be smaller because of their age), occupied by families or other small groups.

This exception is not a big challenge for the thesis that ADUs are better for the environment than SFRs — because no one is building 50-100 year old houses anymore.  Newer houses tend to be much bigger.

Connecting the dots

So, we have a couple of compelling findings that support the idea that ADUs are more or less inherently green.  First, models show that the environmental impact of a dwelling is strongly related to its size.  Second, in the reality of Portland, ADUs are smaller than SFRs, on both an absolute and (usually) per-person basis.

Now, do these findings translate into an actual reduction in real environmental impacts?  The case seems good.  However, we do not yet have confirmation in the form of directly measured end results — for example actual quantities of materials used, energy spent, etc., in real dwellings. The next step for this important topic should be a piece of field research, something that measures the actual environmental impact of real pairs of SFRs and ADUs, both old and new.  (I’m already designing that project in my head. 🙂 )

What about multifamily?

So far I’ve intentionally left out another “infill” option for housing — “multifamily” developments, such as large apartment blocks.  It seems reasonable to presume that such buildings would have relatively small environmental impacts per unit, compared to SFRs, because the spaces are generally small and the building structure allows sharing of physical resources.  However, I have not been able to find any data on this topic. (I’d appreciate any relevant references).

Moreover, it’s clear that big multifamily projects can be controversial in neighborhoods.  Portland has seen hundreds of ADUs installed in neighborhoods with hardly anyone seeming to mind.  I think that’s because ADUs are comprehensible in scale to neighbors, and the reasons they are built, such as “to house my mother-in-law,” are readily understandable.  But when big buildings started going up on SE Division Street, opposition suddenly appeared.   It’s a lot easier for people to see those giant projects as a kind of invasion.

It could be that ADUs are a “sweet spot” in terms of infill development.  Even if the data shows ADUs are not as green as “big multifamily” development, ADUs will likely be quite green compared to the American standard of suburban-sprawl-SFRs.  ADUs add density, but keep people connected to comfortable features and touchpoints like yards and gabled roofs.   In short, ADUs offer a kind of emotional sustainability that might complement the environmental kind.

next week: do ADUs create parking problems?

About Martin John Brown

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

7 comments on “Are ADUs green housing?

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

  2. Eli Spevak
    July 13, 2014

    Interesting that rented single family residences have even less square footage per person (on average) than ADUs. I wonder if one reason for this is the growing number of rental homes occupied by groups of unrelated adults who use every bedroom – whereas more traditional families often have un-used ‘guest’ or ‘bonus’ bedrooms.

    • Martin John Brown
      July 14, 2014

      Yes, I think that’s a factor for sure. Younger single people, in inner SE and NE Portland especially, are renting single family houses and dividing up the rooms. It’s a way to make it affordable and still be in the location they want.

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This entry was posted on July 9, 2014 by in Policy & Trends and tagged , , , , , , .
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