Building Add-Ons are structures that are added to existing buildings in order to increase their size or functionality. The most common type of Building Add-On is an extension, which is a structure that is built onto the side of an existing building. Other types of Building Add-Ons include porches, balconies, garages, and sheds. The term ELLS (4 letters) refers to an Ell, which is a type of Building Add-On that is used to extend the corner of a building. An Ell is usually constructed from wood and is often used to create additional living space.