Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
next door adv по сосе́дству, ря́дом
adj (flat, house) сосе́дний#
next-door neighbour ближа́йший*сосе́д#
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.