The phrase "next door to" is commonly used to indicate proximity or closeness to something or someone. However, there are many other words and phrases that can be used to convey a similar meaning. Some synonyms for "next door to" include "adjacent to," "nearby," "bordering," "neighboring," and "contiguous with." Other phrases that can be used include "in close proximity to," "in the vicinity of," and "in the immediate vicinity of." These variations can help add nuance and variety to your writing, while still conveying the same basic meaning as "next door to".