You can tile over tiles with the right preparation but i would never do this the proper way is to strip right back to original wall finish.
Tile over tile in bathroom wall.
Always stagger your joints when putting new tiles onto old tiles what to do at the edges where double thickness of tiles show.
Very easy diy that will save you hundreds of dollars not paying a contractor.
Grind them down with a grinder.
Tile on tile or as i say it.
Learn the largest tile industry secret to remodel your bathroom for just 200 bucks and 1 days work.
Generally speaking thin set adhesive also known as thin set mortar is great for setting tiles in areas subject to moisture like bathrooms while mastic adhesive is best for drier spaces like.
Roughen the tile with 80 grit sandpaper.
Original tiles on wall the existing tile joints are in yellow.
Then rinse the wall with a water dampened.
If your old ceramic tile is worn or dated you can lay new tile right over the old and avoid the huge job of tearing out the old tile.
Lay a level on the old tile and mark any high spots.
Scrub the wall tiles with diluted degreasing cleaner and a nylon scrub brush.
Don t worry if you do pull a bit of plaster when stripping the tiles as it s easy enough to replace.
And it lasts for 20 years.
Bjd building roofing say.