next door 
adverb
In or to the next house or room: the caretaker lives next door...
- You'll dine like a king and probably want to build a house next door just so you can pop in every day for lunch.
- He took up so much room we moved him to the house next door, which was empty.
- The meeting house will be similar in size to The Fox public house next door and will have parking for 28 cars.
adjective (next-door)
Living or situated next door: next-door neighbours...
- My daughter Rebecca was the only eye witness to the murder of her grandmother by a next-door neighbour in 1995.
- When she was nearly 80, my dear old mum would skip down the garden, jump on to a bench and hop over the wall to check on her next-door neighbour.
- A burglar who twice crept through a roof space to raid his next-door neighbour's house has been jailed for 15 months.
noun [mass noun]
The people, building, or room next door: next door’s dog a man emerged from next door...
- A machine that is so powerful it can suck up dirt not just from your own living room but next door's too.
- The dogs from next door often burrow under the fence and into my garden digging up plants.
- Just as he was getting ready to throw another rock, the dog from next door, Rocky, suddenly appeared.
Phrases
the boy (or girl) next door
next door to