tiling on top of existing floor tiles

[r7mcu]

been asked to retile a floor,but on top of existing tiles.
What is the best kind of adhesive and is there any prep needed?

 

[DHTiling]

Personally i'd rip them up.........

But if they are fixed well and no hollow spots or cracked tiles etc etc...
Then wash the tiles with a degreaser ( sugar soap is good) then use a flexi adhesive and grout...........try to avoid lining your new tile joints up with the old ones...