Can I tile over tile?

Believe it or not - you can. As long as the existing floor tiles are in good condition, and the surface is level, you can install new tile over an existing tiled floor. However, it is not recommended that you tile over high-gloss ceramic or other tiles, as there is insufficient bonding between the old and new surfaces.

Here's how:
The existing tiles need to be thoroughly cleaned with a heavy-degreaser or sugar soap to remove all dirt.
Where applying new tile over existing tiled floors, lightly sand the surface of the old tiles to improve the bonding of new adhesive.
When buying tile adhesive you will also need a tile bonding adhesive liquid, which is mixed into the adhesive to create a strong bond between old and new installations.
Apply tile adhesive over the existing tile floor.
Use an adhesive trowel and ensure that you apply equally over the area to be tiled.
Pay particular attention to grout joints or depressions to ensure a consistent level across the entire area.
Butter the back of the tiles with a thin layer of tile adhesive before placing the new tiles over the old tiles.
NOTE: Buttering the back of a tile simply means to spread a thin, even layer of tile adhesive across the entire back area.
DIY Tips:
When tiling over tiles, use only lightweight ceramic tiles to reduce the load factor.
Remove fractured, cracked or uneven tiles and fill in any voids with a self-levelling agent before applying new tiles.
Make sure that you allow for sockets, outlets or switches that are on the wall.
The increased thickness will mean that these will have to be extended or moved forward.
Doors and skirtings will need to be modified or planed to allow for the increased floor height.
In an open-plan arrangement, edging strips will need to be fitted to allow for the differences in height. You will find these at your local hardware store in the tile section.
If you're not sure about applying new tiles over existing tiles, consider vinyl tiles as an option. These can easily be glued over tiles with contact adhesive.