I've also always assumed it was at Justin's house. Funny thing is, I also always assumed that Daphne's parents have more money. Why? Cause they bought her a car. Yeah, I know, but still, we do get to see Daphne's house for a hot second in S2, the "lame-ass party" scene. Wonder whose room they were in?
I also thought that it was a frat house, but why would Daph throw a party at a frat? And she is SO not sorority material. Not that she wouldn't be an asset to any, but she wouldn't be able to deal with the bullshit that goes along with it.