Just registered, first post... please forgive my ignorance of terms etc.. I'm a DIY just starting my research.

We hired someone to demo our upstairs bathroom in a 1977 built home. Cement "Mud?" floor and all down to what appears to be a 1/2" piece of plywood "base" which the cement/mud floor was originally placed on.

I'm planning on rebuilding the sub-floor with 3/4" plywood (floor rated), and 5/8 cement backer board (mortar & screws etc.) for preparation of 3/8" thick porcelain tile which should be the proper height to have tile flushed with the toilet flange.

My question is regarding the alcove kohler cast iron tub and the sub-floor underneath. I don't believe the cement/mud floor was poured underneath the original tub (missed that part of the demo). I do see the tub had a "leveling stringer" attached to the wall joists.

Not sure if the front of the tub was placed on the edge of the cement/mud floor or if it was placed directly on 1/2" piece of plywood sub-floor, directly over a floor joist.

Anyhow, I'm just wondering... When I'm rebuilding the sub-floor do make the ENTIRE room floor height the same? Do I place the cement backer board underneath the tub area for it to "rest" directly on and then tile up and around the front using silicone between tile and tub?

Thanks a million,
Mike

PS- I realize there maybe a number of other questions needed to assist on the answer. Fire Away. I'm trying to do this correctly but realize that no matter how much reading/research I do... It doesn't replace true experience and I appreciate yours.