
Garrett Q. answered 07/30/19
Experienced teacher in English, the Humanities, and more
Since "falling for" someone or something (in this case, a house) means one is struck by desire for it and wants it immediately, the use of "fall hard," just emphasizes the strength of that desire. So, if "fell for" on its own meant, "He wanted the house," adding "hard" means, "He wanted the house really badly."
Consider a sentence with similar usage, such as, "He tried hard to open the jar, but couldn't." Also, "After the awkward gaffe, the program's ratings plunged hard." The term refers to strength of the action, whether it's wanting a house, twisting a lid, or worsening ratings. I hope that answers your question!