Translation:There are five happy inhabitants in the house.
The word "inhabitant" has the prefix "in-" (meaning "in") built into it already, so using the separate preposition "in" is redundant and somewhat awkward. Grammatically, though, you can't just omit it, either.
That's part of the reason that "resident" is a far better word here, at least in the English. I don't know for sure which word more nearly translates the Finnish one, but "resident" makes for a far less awkward translation.