An apartment to which an extra bedroom has been added through the [subdivision] of a bedroom or living room with a makeshift wall. This usually results in a tiny, rat-maze-like room with no doors, windows, or closets, and no sound [insulation]. Not recommended for adults. A common occurrence in New York City, this procedure is used especially for turning overpriced, small one-bedroom apartments into overpriced, even smaller two-bedroom apartments. Usage is not standard, and some differentiate between converted apartments in which the subdividing wall(s) have already been added, and convertible apartments which have not yet been subdivided. Transparently and insultingly exploiting this confusion, [unscrupulous] (read: all) rental apartment [brokers] almost always classify any overpriced one-bedroom apartment as a convertible two-bedroom.
Renters of convertible apartments are in common [parlance] called suckers, and the act of renting itself is called a mistake.