enclosed area on top of a building. Such a structure may house the top of an elevator shaft, air-conditioning equipment, or the stairs leading to the roof; it can also provide living or working accommodations. Usually a penthouse is set back from the vertical face of a building, thus providing open spaces or terraces on one or more sides; but in recent practice architects and rental agents have referred to the top floor of any building, regardless of setbacks, as a penthouse.

Although the word now denotes a luxurious and expensive apartment, commanding a panoramic view, historically a penthouse was a mere lean-to, or shed, or other small structure attached to a comparatively large building. In medieval times a penthouse, or pentice, was important in siege craft, being the temporary structure that protected besieging forces as they prepared for an attack on the enemy.