A built object is prefabricated when it was constructed by assembling large components which were manufactured off-site. This method of construction saves time and money by standardizing and mass-producing building parts and often by producing them with machines. Only a minimal amount of on-site construction work is required. A prefab structure may be built on top of normally constructed foundations, or it can be a portable construction.

Prefabricated building elements may include large sections of walls, floors, and roofs, or they may be entire room-size or even larger modules. Small buildings such as houses, retail buildings, and fast food restaurants are often fabricated entirely off-site and moved in one piece to their permanent locations. Prefabrication may also be used for larger structures, usually requiring several deliveries to the site.

A building should not be listed as prefabricated when only certain components are prefabricated. The process should include most of its structural and secondary components.