The Chocolate Works in Philadelphia is a former candy factory that was converted into apartments. (Yardi/RentCafe)

Nearly 800 structures were converted to apartment buildings during the 2010s, the highest number in the past seven decades, according to research by data provider Yardi Matrix. Chicago tops the list of U.S. cities with the most adaptive-use apartment buildings, whereas New York City is home to the most converted apartment units in total.

Idle factories have historically been the most popular buildings for conversion into rentals, but office-to-apartment conversions were the most common type in the 2010s.

The 2010s saw 55 times more buildings converted for use as apartments than were converted in the 1950s—a leap to 778 projects from just 14. That number has been increasing rapidly, especially since the 1990s, Yardi Matrix reports. The same upward trend is confirmed by the number of apartment units in repurposed buildings—up to almost 97,000 in the past decade from about 2,000 in the 1950s. In all, more than 240,000 apartments are for rent in large, converted buildings in the United States now.

The United States has many beautiful old buildings—many historic—that are underused or even abandoned and can be repurposed and converted to residential use. In all, 1,876 such buildings—from abandoned dispensaries to vintage gramophone factories—have been converted into apartments since the 1950s, according to Yardi Matrix data.

In addition, the type of building turned into apartments has changed over time. From the 1950s to the 1990s, hotels were the most common type of building converted. That changed to mostly factories in the 2000s, then offices in the 2010s, Yardi Matrix reports.