Housing units are classified by the year the structure in which they are located was built, i.e., the date the original construction was completed (not the date of any later remodeling, addition, or conversion).

In 1960, the categories were: 1959 to March 1960, 1955 to 1958, 1950 to 1954, 1940 to 1949, 1930 to 1939, 1929 or earlier. The 1970 categories were updated by ten years.

Tabulations on the number of units built during a given period relate to the number of units in existence at the time of enumeration, which may not be the same as the original number. Year built data are particularly susceptible to response errors and nonreporting, since respondents must rely on their memory or estimates of persons who have lived in the neighborhood a long time, etc.