Beside and Next door

Beside preposition - In addition to.

Next door and beside are semantically related In some cases you can use "Next door" instead an adjective "Beside".

Next door adverb - At or in or to the adjacent residence.

Beside and next door are semantically related in adjacent topic. You can use "Beside" instead the phrase "Next door" as an adjective or an adverb, if it concerns topics such as next to, close by physically.