Lay new tiles over the old ones?!?

Hello dear experts,

In our living room, we want to have new tiles installed.
At the moment, the floor consists of insulation, screed with water-based underfloor heating, and the old tiles.
Now the following options arise:
1. The tiler wants to simply lay the new tiles on top of the old ones. Is that normal or rather a case of botching? How does that affect the performance of the underfloor heating?
2. Personally, I would have removed the old tiles (but left the screed along with the heating), especially because of the transition to other rooms. But with that option, the tiler says the screed floor would become too uneven (due to tile mortar, etc.) and that the effort to get the surface straight again is not really worthwhile.
3. If the old tiles are to be removed, then the screed would also have to be removed.

What is the proper way to handle this here?

Thank you in advance for your opinions.

Tiling is going well. He is probably right, if you remove the old ones, the screed will also need to be redone; just using leveling compound won’t be enough anymore. What do you want to spend? Karsten

And what about the underfloor heating? Does it still work despite the extra layer, or will the lead time become forever long?

I want to output as much as necessary and as little as possible ;-)

The floor that is currently in place is 30 years old. The tiles are rock solid and the underfloor heating does what it is supposed to do.

However, if I now think that the new floor might be in place for another 30 years, a new underfloor heating would probably make sense...

I can only say what I would do: I would talk to my heating engineer, get advice, and then decide...

If the tiles are laid in thin-bed mortar, they could be removed without damage to the screed. Of course, no one can guarantee that. However, the remaining adhesive must then be milled off and/or leveled over a large area with leveling compound. This is all time-consuming, dusty, and expensive. Talk to the heating installer about the durability of the underfloor heating and have the tiles overflow.

Thank you / The heating engineer was here. The old underfloor heating will last "forever". The decision has been made: The old floor stays in and the new tiles will be laid over it.