We are converting our attached garage into a bedroom/bath. The current garage sits about 3' lower than the rest of the house's main floor. We are going to build the subfloor directly on top of current concrete floor (so it basically will rest on the floor). My question is, how should I prepare the concrete floor for this? I planned on using a water proofing paint and then covering the floor with thick plastic to act as a moisture barrier. I would then lay 2x8 (to give myself enough spacing for plumbing) atop the plastic. I would like to be able to use regular lumber to stay away from pressure treated due to cost, but I didn't know if there was another "best" way.