The Collaborative International Dictionary of English v.0.48:
frat house \frat house\ n. a house used by a chapter of a fraternity at a college. Syn: fraternity house; chapter house. [WordNet 1.5]WordNet (r) 3.0 (2006):
frat house n 1: a house used as a residence by a chapter of a fraternity [syn: chapterhouse, fraternity house, frat house]