Last week, the New York Times asked "Why the Doorman is Lonely," in an article referencing new figures about New York City's oft-referenced excess of un-lived-in apartments. The upshot is that nearly one quarter of New York City apartments aren't used as primary residences, but are pieds-à-terre, or investment properties rented out to tenants.
So, nothing too surprising, but something else in the piece kind of throws off the narrative that these co-ops and condos are primarily left empty by jet-setting millionaires. According to Jonathan J. Miller, the president of the appraisal firm Miller Samuel, "the vast majority of pieds-à-terre are middle class... owned by people who have a studio in the city and a home in the suburbs, or maybe it was their first apartment that they chose to keep and rent it out."
I suggest reading that Times article because it has some pretty interesting info about how people evade property tax.