While there is a lot of recent talk about converting hotels and offices to residential uses, John Cetra, cofounder at New York-based architecture firm CetraRuddy. says New York City has been at the forefront of residential conversions for almost 50 years, after passing regulations that make the process easier.

When Cetra started New York-based architecture firm CetraRuddy thirty years ago, he says there were many more buildings available for conversions.

"Most of them were in the financial district and were not fully occupied at that time for different economic reasons," Cetra says. "People were leaving the city for different reasons. So there were a lot of office buildings built from 1910 to the 1930s that no longer met the needs of the modern, contemporary office environment [at the time]."