A report from RentCafé looks at which types of properties developers have most often repurposed as apartments. : Around 2,000 old buildings in the U.S. have been converted into apartments over the past 70 years, with 778 of those conversions taking place in the last decade, according to a new report from RENTCafé. That figure represents an all-time high for multifamily conversions. In the 2010s, offices became the most common building type to be turned into apartment rentals, with factory conversions dominating the 2000s while hotel to multifamily adaptive reuse projects were the most popular from the 1950s through the 1990s.