Are you sure of what you're saying?! With the loads you're talking about, putting them on blocks isn't a good idea. You ought to have poured footing/pilings/whatever but NOT blocks. If you're building the room now, talk to the installer about having him do it now. It's probably cheaper while he's there. Yes, a Warm Floor can be done by yourself but it might have less 'limitations/problems' during the install to have it done before the floor goes down. Most warm floors go under the flooring unless so they can be 'gotten to' if the need arises. Unless you have a basement or crawlspace under the room, you'll have to do the install after-the-fact. This will make the room 'that much higher'. Doors may have to be cut, trim may have to be cut or re-installed, and the transition into the main house may be higher. At worst, have the room built by the contractor with the anticipation of having a warm floor installed at a later date. That way, you're one step ahead.