A building's extension is a wing. As in, "The hospital is adding a wing".
Or, "They named it 'The Gonzo Wing' because Gonzo funded it".
Structure, Construction, Edifice, or Erection.
Other words for structure are composition, arrangement, shape, form,or building.
erecting ;) also: structure or constructing based upon the definition of building.
These are synonyms for the word bridge: arch, bond, branch, catwalk, connection, extension, gangplank, link, overpass, platform, pontoon, scaffold, span, tie, transit, trestle, viaduct and wing.
The noun 'annex' is a word for a building added on to a larger one; an auxiliary building situated near a main one; an addition that is made to a record or other document, an appendix. The word 'annex' is also a verb, meaning to take control of a country or region by force; to add as an extra or subordinate part of something.
Another word for outhouse is privy.
An annex is an extension of or an addition to a building. A small room off of a main room is an annex, and attics are another type of annex.
extensors / extensor muscles
tug
Building is another word for barn. Additional synonyms include shed and stable.
a tower, a house, a shelter...
Edifice
Barn or shed.
extension to obey
You can plug an extension cord into another extension cord. A extension cord is to help make doing something easier.
Lodge , Fellowship Hall ,
A building extension is typically referred to as an "addition" or "annex." It involves expanding the existing structure to increase living or working space.