I would recommend not using subtraction brushes and just build all with additive. It allows for you to quickly change something if its not working out because you don't have to worry about moving two brushes. Also ive seen Max and Maya do strange things with subtraction brushes when importing in level blockouts.
I do want the current room to be hollow, but when I subtract inside of it, I lose the floor. It's the reason I posted this thread. It all works if I build the room above the ground floor, but I want the room to share the same floor as the ground plane, and subtracting space from within the room causes the floor to go too.
I was planning to subtract BSP from within the BSP I added for the room itself... I haven't heard of anyone using a brush for each wall. Is this a standard practice?
This is how BSP works. You put a block on a block, and this is the inside of solid space. To get a building, use a brush for the floor, each wall and ceiling. OR if you want that current room to be hallow, add a subtraction brush inside of it