Glass Rooftop Penthouse, London, United Kingdom

In architecture, the term penthouse is used to refer to a structure on the roof of a building that is set back from the outer walls. These structures do not occupy the entire roof deck. High-rise buildings will often have penthouse structures enclosing mechanics such as those in an elevator machine room.
While European designers and architects long recognized the potential in creating living spaces that make use of rooftops and such setbacks, in US cities, exploitation of these spaces began in earnest in the 1920s. It was a matter of news when the development of a rooftop apartment at the Plaza Hotel overlooking Central Park was announced in 1923, and this was followed by rapid development of luxury penthouse apartments in the following years.
When used as private outdoor terrace spaces, setbacks can allow for significantly larger and more protected spaces than cantilevered balconies. Due to the desirability of this outdoor space, buildings may be designed with such setbacks on more than one of its uppermost levels, to allow apartments on several levels to feature such terraces. Not all penthouses have such terraces, but they are a desired feature. One such space may be divided among several apartments, or one apartment may occupy an entire floor. A penthouse apartment/condominium may also provide occupants with private access to the roof space above the apartment, instead of or in addition to terrace space created by an adjacent setback.