A building extension is an addition to an existing structure, usually to increase the size or functionality of the building. The crossword solutions for this term are ELL (3 letters), ANNEX (5 letters), WING (4 letters), and ELLS (4 letters). An ELL is a WING or extension of a building that projects from the main structure at an angle. An ANNEX is an additional building or part of a building that is connected to the main structure. A WING is a section of a building that projects out from the main structure, usually at a right angle. ELLS are plural for ELL.