yes its easy to tile over tiles, but first you must find out what your substrate is, then work out an approx. weight of the old tiles, adhesive and grout per sq mtr, then work out the weight of the tiles you plan to use plus the adhesive and grout, add them both together, that would give you an approx. total weight per sq mtr, then check against whatever your substrate is capable of holding. that's before doing ANYTHING. If all is ok, give the tiles a good scrub down with sugar soap or a good degreaser, set your lines out then start tiling, check the size of your new tiles to determine wether you can use tubbed or have to use powdered addy