Year built refers to the date the original construction of the structure was completed, not to any later remodeling, addition, or conversion.
The figures on the number of units built during a given period do not necessarily represent the number of housing units added to the inventory during that period. The figures represent the number of units constructed during a given period plus the number created by conversions in structures originally built during that period, minus the number lost in structures built during the period. Losses occur through demolition, fire, flood, disaster, and conversion to nonresidential use or to fewer housing units.