A building addition is a structure that is added to an existing building. The most common terms for a building addition are 'ELL, ' 'WING, ' 'EXTENSION, ' and 'ANNEX.' An ELL is a structure that extends from the main building in a perpendicular direction. A WING is a structure that extends from the main building in a parallel direction. An EXTENSION is a structure that is added to the back or side of the main building. An ANNEX is a structure that is added to the main building, but is not connected to it.