Adjoining, a.
Joining to; contiguous; adjacent; as, an adjoining room. "The adjoining fane." Dryden. Upon the hills adjoining to the city. Shak.
Syn. -- Adjacent; contiguous; near; neighboring; abutting; bordering. See Adjacent.
Shades of meaning
Fernald has no entry of its own for adjoining; he lists it under adjacent.
Where he writes about it:
To add is to increase by adjoining or uniting: in distinction from multiply, which is to increase by repeating. — from add
Adjacent farms may not be connected; if adjoining, they meet at the boundary-line. — from adjacent
Contiguous may be used for either adjacent or adjoining. — from adjacent
Buildings may be adjacent or adjoining that are not attached. — from adjacent