National analysis · 3,144 counties · Census ACS 2020-2024

Before 1940

16,880,456 American homes still standing were built before 1940 — 11.7% of the country’s housing. They are not spread evenly. Half of them sit in 89 of the 3,144 counties, and in St. Louis the share reaches 57.9%. Here is where they are, county by county.

Concentrated, not scattered

Half of America’s pre-1940 housing is in 89 counties.

There are 143,775,355 housing units in the 50 states and the District of Columbia, and 16,880,456 of them were built in 1939 or earlier. Sorted by how many each county holds, the first ten counties account for 20.1% of the total, the first fifty for 41.2%, and the first hundred for 51.9%. The other 3,044 counties share the rest.

The ten counties with the most housing units built before 1940. Margin is the Census Bureau’s 90% margin of error on the count.

Cook County, Illinois leads with 632,386. Los Angeles County is second by count, 522,483, even though only 14.3% of its housing is that old: it is simply very large. The share and the count tell different stories, and a reader quoting either should say which one they mean.

Where the share is highest

Old cities, and a band of small counties in New York and Pennsylvania.

Ranked by share, among the 1,773 counties with at least 10,000 housing units, two kinds of place lead. One is the old cores of eastern and midwestern cities: St. Louis city, Missouri at 57.9%, Kings County (Brooklyn) at 47.0%, Boston’s Suffolk County, Manhattan, Baltimore and Philadelphia. The other is a run of smaller counties across upstate New York and Pennsylvania, where a large share of the housing predates 1940.

Counties with at least 10,000 housing units, by share built before 1940. St. Louis and Baltimore are independent cities that the Census counts as counties.

Counties with at least 200,000 housing units, by share built before 1940.

Across all 3,144 counties, 582 have a quarter or more of their housing from before 1940.

By state

From 30.5% in Massachusetts to 1.1% in Nevada.

All 50 states and the District of Columbia (in italics), by share of housing units built before 1940. Median year built is the Census Bureau’s own estimate (table B25035).

How old is the average house in the US?

The median American home was built in 1980.

The Census Bureau puts the median year built for all US housing units at 1980, with a margin of error of 1 year. Half of the country’s housing is older than that and half is newer. In the decade counts behind this study, 49.7% of housing units were built before 1980, 25.6% before 1960 and 16.2% before 1950.

By state, the median runs from 1959 in New York, the oldest of any state, to 1996 in Nevada. The District of Columbia’s median is 1958. In St. Louis city the median falls in the Census’s bottom bracket, 1939 or earlier: more than half its housing predates 1940.

One date matters for buyers in particular. Lead-based paint was not banned in US residential use until 1978. The Census decade brackets end at 1979, so the 49.7% figure for housing built before 1980 is the closest this data comes; it is not a count of homes built before 1978.

What the year tells a buyer, and what it doesn’t

A reason to ask, not an answer.

The age of a building is the first clue an inspector uses about what may be inside it. Three of the era checks in the Before Regret report engine reach back past 1940:

• Knob-and-tube wiring was standard in US homes built before roughly 1950. (what knob-and-tube wiring is)
• Homes built before the early 1970s commonly used cast iron for drain, waste, and vent piping. (why cast iron pipes corrode)
• Lead-based paint was not banned in US residential use until 1978.

None of that is a finding about any particular house. The Census measures when a building was first constructed, not when it was rewired, re-plumbed or renovated, and many of these homes have been updated more than once. A pre-1940 house is a reason to ask the seller and the inspector specific questions, and to look up the permit history by address to see what work was recorded.

When was my house built? How to find out by address

Census data cannot tell you. Three records can.

Everything above describes counties. For one house, the year comes from local records:

• The assessor or property appraiser’s parcel record. Search the address on your county assessor’s or property appraiser’s website and find the year built field. Counties do not all define it the same way. Sarasota County, Florida’s property appraiser defines Actual Year Built as “the year an improvement is completed.” Gunnison County, Colorado’s assessor records Original Year Built as “the year the construction permit was initiated.”
• Not the effective year built. Some records show a second date. Sarasota describes Effective Year Built as an appraisal judgment of a structure’s condition and utility; in Gunnison it stays the same as the original year “until there are significant updates or remodels that extend the life of the structure.” Either way, it is not when the house was built.
• The building permit history. The original construction permit, where one survives, dates the house, and later permits show what has been rewired, re-plumbed or added. Here is how to look up building permits by address in any county.

Once you have the year, a free Before Regret report uses it to list the checks that matter for a house of that era in that county.

Method

What was counted.

Sources are tables B25034, Year Structure Built, and B25035, Median Year Structure Built, from the U.S. Census Bureau’s American Community Survey 5-year estimates for 2020-2024, retrieved from the Census API for every county. The variable labels were checked against the Census metadata for that release, and each county’s decade counts were checked to sum to its total. Housing units include occupied and vacant units, and an apartment building counts once per unit, not once per building.

The 3,144 counties and county equivalents of the 50 states and the District of Columbia are included. Puerto Rico’s 78 municipios are in the same Census table and are left out of every figure here, because the question is about the housing stock of the states. Share rankings consider only counties with at least 10,000 housing units, so that a very small county with a large margin of error cannot top a list.

Limitations

What this does not show.

The Census Bureau warns about this question in its own subject definitions:

Data on year structure built are more susceptible to errors of response and non-reporting than data for many other questions because respondents must rely on their memory or on estimates by people who have lived in the neighborhood a long time.

Year built refers to when the building was first constructed, not when it was remodeled, added to, or converted, and the counts are of buildings still standing when the survey was taken. Every figure is an estimate with a margin of error; 149 counties have a margin on the pre-1940 count larger than half the estimate itself, and the data file flags them.

Questions this data answers

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