Learner's definition of NEXT DOOR
in the next house, apartment, room, etc.
The people (who live) next door own a very large dog.
We've lived next door to [=next to] each other for the past 30 years.
He/She seemed like the boy/girl next door. [=like a wholesome young man/woman from a middle-class family]
next to your or someone else's house, apartment, room, etc.
He bought the house next door.
I'll be staying in the room next door.
Go next door [=to the house/apartment next door] and ask for a cup of sugar, please.
in a place that is very close to something else