addition
An attached section constructed after the original building or structure had been completed.
An addition is a section of a building or structure which was built after the original structure was completed, and which was attached to the original.
Both the later construction and attachment to the original are part of the relationship. The new section extends an already completed object.
A projecting wing has a geometric distinction, but one built separately after completion has this addition meaning. A detached structure does not become an addition merely because it stands nearby.
