next door to
1. Literally, in a house, office, room, building, etc., adjacent (to someone or something). We lived next door to a train station, so we know what it's like having to deal with loud noise all day long. It's surreal having an office right next door to my old professor.
2. By extension, very close to (some state, outcome, etc.). It breaks my heart seeing my grandfather like this, bedridden and living next door to death. Success is next door to self-confidence—the less you believe in yourself, the less successful you will be!