| We just finished doing two bathrooms in our house - I'm not a tile expert so keep that in mind:
I would not put tile over tile - you'll end up with a ramp effect from the adjoining hallway - seating the toilet will be a real challenge - you don't know how well the previous job was done.
If it's a small bath taking up the old stuff will be less of a challenge and you'll know it's right when you finish. If handi backer was put down maybe you'll get lucky and the previous installer nailed it down without thinset under it.
If you gut it make sure you use thinset under the new handi backer and install screws at each point indicated. The bath we just finished we put travertine down over 1/4 backer. Granted travertine is thick but even that height created a toilet seating challenge. It took two attempts with a tile wax seal and another plain wax seal stacked on top of it. Helpful tip on seating the toilet - cut a drinking straw in half and slide them down over the bolts - gives you something to guide the toilet down over. Good luck with your project.....  |