A penthouse is a unit on the highest floor of a building – usually an apartment building, condo, or hotel. The word ‘penthouse’ was originally used to describe a house built on the roof of an apartment building. Today, the word is also used to refer to apartment units that are located on the top floor of a tall building. Penthouses originate from the 1920s, when New York City experienced high economic growth. Because of this, people got wealthier, and more wanted to live in the heart of the city.