Tiling over existing tiles is legitimate and tilers do it regularly. It saves the mess, time and cost of removal. But it only works if the surface underneath genuinely is sound, and it introduces two constraints that catch people out.
Test the existing tiles first
Tap every tile with the handle of a screwdriver or your knuckle. A well-bonded tile gives a solid, dull note. A tile that has lost its bond sounds hollow and drummy.
Any hollow tile has to come off. Tiling over it means your new tile is bonded to something that is itself only loosely attached, and the whole assembly comes away eventually, usually taking the substrate with it.
Also check for cracked tiles, which often indicate movement behind, and for tiles that are loose at the edges. If more than a small proportion sound hollow, take the lot off. Patching a few and tiling over the rest gives you an uneven surface and an unpredictable bond.
Preparation is what makes it work
Old glazed tiles are smooth and non-porous, which is the opposite of what adhesive wants. Two things are needed.
Key the surface, either by abrading it mechanically or, on a small area, by scoring the glaze. Then prime it with a primer intended for non-porous surfaces, which gives the adhesive something to grip.
Skipping this is the most common reason tile-over-tile fails. The new tiles look fine for a year and then start sounding hollow.
Clean thoroughly as well. Bathroom tiles carry soap residue and years of cleaning products; kitchen tiles carry grease. Both defeat adhesive.
Constraint one: weight on walls
Every wall substrate has a maximum weight of tiling it can carry, and adding a second layer can exceed it.
Plasterboard carries considerably less than a sand and cement render, and the limits are set out in the relevant British Standard. As a rough guide, plaster and plasterboard are the substrates most likely to be overloaded by doubling up, particularly with heavy porcelain or stone.
Ask your tiler to check the combined weight against the substrate, and check the adhesive manufacturer's data. If you are moving from light ceramic to porcelain or natural stone, the original wall may not take both layers.
Constraint two: floor height
On floors, a second layer of tile plus adhesive typically adds 15mm to 20mm. That is enough to matter.
Doors may no longer clear the floor and need trimming from the bottom. Thresholds to adjoining rooms become a step. In a bathroom, the height relative to the shower tray, the bath panel and the WC soil connection all change.
Check the door clearance and the thresholds before committing. It is a solvable problem but a much cheaper one to solve on paper.
When to strip instead
Take the old tiles off if a significant number sound hollow, if the surface is badly uneven, if there is any sign of damp or movement behind, or if you are tiling a floor where the added height causes problems.
Removal is charged by time. MyJobQuote puts tile removal at around £30 an hour, and it is dusty, noisy work that frequently damages the plaster underneath, so allow for making good.
In an older South London property where the tiles may be sitting on original lime plaster, stripping often reveals that the wall needs replastering or boarding before anything can be tiled at all. That is worth discovering before you have bought the tiles.