Can someone help explain why the sentence needs both 'inn' and 'i'? My best guess is the verb for 'enter' is "gå inn" and "i huset" is an adverbial clause?
"inn i" is a phrasal preposition indicating motion inward, similar to "into" in English, which when you think about it also consists of two parts "in" and "to".
Would "Brannmannen går i huset" or "Brannmannen går inn huset" work as well/sound natural?
Inn is an adverb only and cannot be used as preposition. "i" only looks more like "walks in the house", not "entering the house"