Housing Units Built by Year
Data on types of housing units built by decade
When reading the Washington Post story Aging homes are crumbling across the U.S. Should government pay to fix them? (9/6/2025) with the subtitle, The average age of a U.S. home is now 40 years old. Homes tend to be the oldest in the Northeast, Mid-Atlantic states and along the Appalachian Mountains, I noticed that using a mean here is likely misleading, as historic homes would be outliers that increase the average age of a home. This should alert us to the data rule: Know the distribution, not just the mean and median. This article got me curious about the distribution of the age of housing units.
Let’s go to the data, which is from the Census Bureau’s American Housing Survey. Figures 1 and 2 are the same data but grouped differently. In Figure 1 the data is grouped by the year the housing unit was built, with the bars colored by housing type. Caution: All groups are 10-year tallies except the first, which is all units built before 1939. What do you see in the graph?
The most obvious thing is that we build more single-family detached homes than any other type by far. This trend isn’t entirely surprising, as housing units with 50 or more units are bigger and, in some years, contribute more housing units, apartments, than single-family homes.
Starting around 1950, the U.S. population growth is close to linear, which means the increase in population is approximately constant, implying that we would add about the same number of housing units each year. In fact, it looks like a slight decrease in most housing unit types, but here Figure 1 is hard to read, and it would be better to group by housing unit type. Hence Figure 2. It is the same data but grouped by housing type, and the color of the bars represents the decades. It is worth taking a moment to notice how Figures 1 and 2 tell different stories.
Focusing on single-family homes, I would not have expected no increase in units built by decade. Considering that the baby boomers were born from 1946 to 1964, I would expect a jump in housing about 20 years later, or roughly in the 1970s and 1980s, and we don’t see that in single-family homes. We do, however, see that in other types of housing units.
For most types of units, the peak decade of building was the 1970s or 1980s, so this does match when the boomers would be needing housing. The exceptions are single-family detached homes, which saw a record in builds in the 2000s. Another exception is the 50 or more units, which have been increasing in builds for the last few years, with the 2010s being the record years.
I’m not sure what this all means, but I would expect more housing units being built, but this is a very superficial view. Maybe what we are seeing is boomers moving out of their homes into apartments, so we don’t need more single-family homes but more apartments, for example. This data is a good start at understanding housing as related to needs, but the modeling needs additional information, especially the age of cohorts and preferences in housing types. Still, the dropoff in the 2010s for the smaller units is surprising. What am I missing?
One other fun graph is Figure 3, which breaks down the type of single-family homes by the basement type. The concrete slab, with no basement of any type, grew over the decades to be the predominant type. I wonder if this has something to do with the locations and soil types of the homes built. Basements seem like an excellent thing, but they do entail extra cost. SStill, it seems that basements would be more desirable.
As to the Washington Post article, they do have a point about the number of older homes that will need expensive maintenance. Effectively we have a society-wide deferred maintenance problem. By just eyeballing the graphs, about half the houses were built before 1980, all of which are now 45+ years old. This creates a problem because we need to build newer housing units as the population grows while also requiring major repairs to older homes. I see this as an issue of our growth model. GRowing works well until the need arises to grow and repair or replace older infrastructure. We’ll see how that goes, possibly not well.
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I am a tenured mathematics professor at Ithaca College (PhD in Math: Stochastic Processes, MS in Applied Statistics, MS in Math, BS in Math, BS in Exercise Science), and I consider myself an accidental academic (opinions are my own). I'm a gardener, drummer, rower, runner, inline skater, 46er, and R user. I’ve written the textbooks “R for College Mathematics and Statistics” and “Applied Calculus with R.” I welcome any collaborations, and I’m open to job offers (a full vita is available on my faculty page).