shower stall kits
 

Hi all, was checking out the shower kits at home depot the other day and was wondering if these kits go over your existing tile or do you need to remove them. thanks

Most times you attach the unit to the studs, backer board up to the flange and tile over the flange.

You need something to attach the stall to, like the studs Neil suggested. The flanges are usually screwed or nailed to the 2x4's and then the edges finished off.
Unless of course you are talking about the glue up plastic walls. These could be attached over the tile IF you can find an adhesive strong enough to stick to the tiles.