I just read the following sentence

> 
> Les voisins habitent dans la maison d'à côté
> 
> 
> 

I couldn't understand why “maison” has been used in singular. Could you point out some rules with regard to plural formation?

Also, how would you say (in the sense of definition of neighbours) "Neighbours live in houses close by/next to each other" altering the above sentence minimally (just to understand the nuances).
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Here, "les voisins" refers to a family, not to multiple separate neighbors living in separate houses, hence the singular.
Answering the second part of your question:

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> Les voisins habitent dans des maisons proches les unes des autres.
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> 
>