Should new tiles be fitted on top of existing ones or should they be removed first?

depends how secure they are ..would always recommend to remove if you can ,,but if they are secure then you nedd to key them up with some glass paper then pva seal before tiling ,,good luck

I woud'nt recommend laying any tile on top of another, although with modern adhesives you may get away with it. but if you do make sure the new floor tiles are offset from the old ones and rough up the old ones surfaces to key the new ones also it gets rid of any cleaning agents used over the years/ good luck ... capricorn ceramic tiling

Remove existing tiles, then prepare surface for new tiles. Martyn

They shoud always be Removed , and 90% of jobs if its on plasterboard you have to change then too

you can do it that way,there is adhesive for just that purpose,never done it mind you,but i think whats the problem with doing it that way.i can see none. it would help make extra waterproof. chris

It is possible but, It does depend on a number of factors. The tile finish/ condition and how well they are already attached to the existing floor, what type of base they have been laid on, whether the additional weight is going to cause issue, height differentials in the flooring etc. Personally, I would always remove these where possible as in my experience it allows me to know what i am dealing with from base up and creates a better look if you don't have a larger step down in floor levels.

In my working lifetime i have layed thousands of metres of tiled floors,many of them over existing tiles.As long as the existing floor is solid and given a good coat of bonding agent such as p.v.a you should not have any problems.Also use a good adhesive preferably a powder that you mix yourself not that pre-mix rubbish. Hope this helps