To me, a 'house' is not a roof and a bunch of walls with a 'Gone with the Wind' decor....rather it's the brother/sisterhood that you have on a local and inter/national level!
If I'm not mistaken, NPC sororities do not have pledge houses. As stated in another thread, the pledge house was an idea created by the producers of "Sorority Life".
My chapter doesn't have a gigantic mansion that we can call home, but that doesn't mean we lack the sisterhood that housed chapters have.