I. ˌnext ˈdoor¹ adverb

1. in the house, room etc next to yours or someone else’s:
the boy next door
Her office is just next door.

2. next door to something next to another building, room etc:
They live next door to the shop.

[TahlilGaran] Dictionary of Contemporary English ▲

II. next door² noun [uncountable] British English informal
the people living in the house or apartment next to yours Synonym : neighbour:
Have you seen next door’s new car?

[TahlilGaran] Dictionary of Contemporary English ▲

ˈnext-door adjective [only before noun]

1. next-door neighbour the person who lives in the house or apartment next to yours

2. next-door apartment/office etc the apartment etc that is next to yours

[TahlilGaran] Dictionary of Contemporary English ▲

next to preposition very close to someone or something, with no other person, building, place etc in between:
I sat next to him at dinner.
The hotel was right next to the airport.
beside preposition next to the side of someone or something:
Ella came and sat down beside me.
They were sitting beside the pool.
by preposition next to something – often used about being very close to a window, door, or the edge of something such as an area of water:
I saw him standing by the window.
Weymouth is a pretty little town by the sea.
She lives by the river.
next door adverb in the building or room next to yours, or next to another one:
The house next door is much bigger than ours.
Have you met the people who’ve just moved in next door?
alongside adverb, preposition close to the side of something, especially a river, railway, boat, or vehicle:
I decided to take the path alongside the railway track.
A police car pulled up alongside.
adjacent adjective, adverb formal a building, room, or piece of land that is adjacent to another one is next to it:
They walked through a rose garden adjacent to the hospital.
The blaze spread to two adjacent buildings.
adjoining adjective formal an adjoining room, building, or piece of land is one that is next to another one and is joined to it:
We had adjoining rooms at the hotel.

[TahlilGaran] English Thesaurus ▲

next door
adv. or adj.
1. In or to the next house or apartment.
He lived next door to me.
She telephoned next door to ask about John.
The house next door caught fire.
2. Very close.
■ Used with "to".
The sick man was next door to death.
Printing secrets about our country's missiles is next door to treason.

[TahlilGaran] English Idioms Dictionary ▲