You can tile over existing tile, and it is standard practice in some situations. The new tile inherits the old installation's flatness, bond and movement, so the question is whether the existing floor is sound enough to be a substrate.
Four conditions, all required
1. Everything is well bonded. Tap across the whole floor with a coin or a screwdriver handle. A hollow sound means the tile below has lost its bond and it will take the new tile with it. Any hollow area has to come out.
2. The surface is flat. Existing lippage becomes new lippage — worse, because you cannot bed a tile into a hump. Check with a straightedge: within 1/8 in over 10 ft for large format.
3. There are no cracks. A crack in the old floor is either a substrate crack or a movement crack, and it will telegraph through. An uncoupling membrane over the old tile handles small ones; a structural crack does not get tiled over, it gets diagnosed.
4. The height increase is acceptable. This is the one that actually stops most jobs.
The height problem
Old tile plus new thinset plus new tile is typically 1/2 to 3/4 inch. Consequences, in the order people discover them:
- Doors no longer clear the floor and must be cut down
- Appliance openings — a dishwasher may no longer slide out under the countertop, and that is a kitchen you cannot service
- Toilet flange now sits below floor level and needs an extender for a proper seal
- Transitions to adjacent rooms become a trip step
- Cabinet toe kicks and baseboards get shorter
Measure the dishwasher opening before deciding. It is the single most common reason a tile-over job has to be undone.
Preparation
Existing glaze is smooth and non-porous, so the bond depends entirely on the mortar.
- Scuff-sand or acid-etch the glaze, or use a bonding primer made for the purpose
- Clean thoroughly — a tiled floor has years of wax, soap and grease on it
- Use a modified thinset rated for tile-over-tile bonding
- Offset the new joints from the old ones so a new joint does not sit over an old one
When it is always wrong
Over a shower floor. The waterproofing is under the existing tile and cannot be inspected. Any failure below stays hidden and keeps working.
Where the existing floor is on an unknown substrate. Adding weight and thickness over something you have not seen is a structural guess.
Where there is any moisture history. A stain, an efflorescence bloom, a musty smell — that is a reason to open it, not to cover it.
The honest comparison
Demolition is a day of noisy, dusty work and a skip. Tiling over saves that day and costs you the ability to inspect what is underneath, plus half an inch of height forever.
On a small bathroom floor in known-good condition, tiling over is fine. On anything larger or anything uncertain, the day of demolition is the cheaper day.