next-door neighbor
A person who lives or works in the accommodation immediately adjacent to one. My next-door neighbor offered to look after my pets while I'm away. We have new next-door neighbors in our office building. I think they said they're doing financial consulting work for small businesses.
one's next-door neighbor
the person living in the house or apartment closest to one's own. My next-door neighbor came over to borrow a shovel. I will be visiting our next-door neighbor if you need me.