A building add-on is an additional structure that is added to an existing building. The crossword solutions for this term are ELL (3 letters), WING (4 letters), ANNEX (5 letters), ELLS (4 letters), and EXT (3 letters). An ELL is a short extension of a building, usually at a right angle to the main structure. A WING is a section of a building that extends from the main structure. An ANNEX is a separate structure that is attached to a larger building. ELLS are two or more ELLS connected together. An EXT is an abbreviation for extension, which is an addition to a building.