A basement is one or more floors of a building that are either completely or partially below the ground floor. Slab-on-grade buildings do not have basements. Basements are typically used as a utility space for a building where such items as the furnace, water heater, car park, and air-conditioning system are located; so also are amenities such as the electrical distribution system, and cable television distribution point.

In British English the word 'basement' is used for underground floors of, for example, department stores, but is rarely used for a space below a house, and the word cellar is used to apply to any such large underground room. Subcellar is a cellar that lies further underneath.

Historical development

For most of its early history, the basement took one of two forms. It could be little more than a cellar, or it could be a section of a building containing rooms and spaces similar to those of the rest of the structure, as in the case of basement flats and basement offices.

However, beginning with the development of large, mid-priced suburban homes in the 1950s, the basement, as a space in its own right, gradually took hold. Initially, it was typically a large, concrete-floored space, accessed by indoor stairs, and with exposed columns and beams along the walls and ceilings, or sometimes, walls of poured concrete or concrete cinder block.

Types of basement by structural design

A "walk-out" or "daylight" basement is a modern architectural form where the house is situated on a slope and part of the basement is above ground. Occupants can walk out at that point without having to use the stairs. For example, if the ground slopes downwards towards the back of the house, the basement is at or above grade (ground level) at the back of the house. It is a modern design because of the added complexity of uneven foundations; where the basement is above grade, the foundation is deeper at that point and must still be below the frostline.

In a "look-out" basement, the basement walls extend sufficiently above ground level that some of the basement windows are above ground level. Where the site slopes gently and is insufficient for a walk-out basement, a look-out basement will result. Sometimes, a look-out basement is deliberately constructed even on a flat site. The advantage is that the basement windows are all above grade. The disadvantage is that the main floor entry is above grade as well, necessitating steps to get up to the main floor. The raised bungalow design solves this by lowering the entry half-way between the main floor and basement to make a dramatic, high-ceiling foyer. It is a very economical design because the basement is shallower, and excavation costs are minimized.

A “walk-up” basement is any basement that has an exterior entrance via a stairwell. Some designs cover the stairwell with angled “basement doors” or "bulkhead doors" to keep rain water from accumulating in the stairwell.