This subject has been covered before so it's worth googling groups for.
The consensus appears to be "yes it's OK but a bit of a cheat".
I am finally coming to the end of my bathroom tiling/fitting/extractor
install/spot light fitting etc etc, and have, for the most part, tiled on
My reason for not removing the old tiles relates to the construction of the
upstairs walls in our property (2 plaster board leaves with egg crating in
the middle!). Had I attempted to remove the tiles from these I would almost
definitely have ended up re-modelling the entire upstairs space.
Points to consider are:
Are the existing tiles seriously fixed in place and level?
Preparation - the existing tile faces must be clean and de-greased. I
actually attacked mine with a grinder putting 3 grooves in each tile face to
Architraves and skirting - I replaced both and packed the architrave out by
the thickness of the tile + adhesive. I used the packer to form a double
rebate look which is OK and I think preferable to leaving what appears to be
just a thick architrave.
Adhesive quality - I was recommended to get decent adhesive (BAL White Star)
for this application.
Batons - fixing batons to the wall to tile to is more fun. I ended up
fitting "legs" below the baton and packing to the correct height.
Untiled areas - before starting the re-fit, I remember chatting to a work
mate about some aspect of the job and noticing that as he leant on one of
the walls, the existing tiles appeared to be moving! Turned out that the
top half of the room had been tiled some time after the bottom half and the
tiles had been applied on top of wall paper (scary). So, when I got the
ceiling skimmed, I got the top half of the wall skimmed to level with the
face of the tiles on the bottom half. Also any other untiled areas (eg
behind the sink) were also skimmed.
Space - be prepared to lose 20mm off room dimensions obviously.