In general when describing a person's location we say they "are in" or "are at" some place. "She is at Aunt Sally's house." "He is in the store." "He is at work." Etc.
If we are using a preposition to describe the place, either as a separate word or part of a compound word, we do NOT use "in" or "at". In this case we don't need two prepositions. "He is downstairs." "She is overseas." "The treasure is under the hill."
"Next door" is something of a special case, an idiom. It does not take "in" or "at". We say, "She is next door." (While "next" is generally classified as an adverb or adjective, arguably here it is being used as a preposition. Idioms can be difficult to classify.)