noun
Used as a noun, often with particles like が, を, に, の. Emphasizes individual houses one by one.
一軒一軒の家に花が飾ってあった。
Flowers were displayed at each and every house.
彼は一軒一軒を丁寧に調べた。
He carefully inspected each house.
adverb
house to house; door to door
Used adverbially, often with verbs like 回る, 訪ねる, 配る. Describes going from one house to the next.
新聞を一軒一軒配った。
I delivered newspapers from house to house.
警察が一軒一軒聞き込みをしている。
The police are making door-to-door inquiries.