Make sure condition and the surface is solid and level. however, there are some things to consider before you start:
check the tiles
make sure the existing tiles are stable, dry, and securely bonded to the wall or floor. you can test the strength of the tiles by gently tapping them with a wood mallet. if they sound hollow, they are not strong enough to support new tiles. you should also check for cracks, chips, or loose tiles.
check for moisture
make sure the surface is free from moisture, mildew, or dampness. if there are moisture issues, you should address them before you start tiling.
check the background
the background should be able to support the weight of the new tiles. in older or poorly constructed buildings, adding extra weight to the structure could impact its stability.
check the height
adding new tiles over existing ones could affect the height of the floor or wall, which can impact doors, cabinets, and appliances.
tiling over existing tiles can be a cost-effective and diy-friendly way to update your space. however, if the existing tiles are not in good condition, you may need to fix or replace them before you can start tiling.