500-year-old Jura marble floor... what to do with it?

In the past, something like this was placed in thick-bed mortar. It can also compensate. But I can imagine that hardly any tile setter learns or can do this anymore ;)
Just ask.

And then definitely not in a country house style – a beautiful minimalist kitchen or guest WC would be perfect :)
You could also make it as a tile panel: simply move the kitchen forward a bit by the thickness of the material.
Do you possibly have a solid wall where you can do without a few centimeters?

The kitchen will actually be very minimalist....but my wife does not want tiles there. A tile backsplash is not possible because the kitchen has a large window as a "backsplash," otherwise there is no wall in the kitchen at all. If at all, only the entrance area remains, or I might come up with something else nice. Guest toilet also sounds tempting, but at the same time probably even more complicated than the entrance area.

We faced a similar decision. We were given old marble in various sizes up to 2 x 2 meters (color green...). Thickness 25mm and 30mm. It would have been laid directly in the heated screed. It was all no problem and even already planned, including installation pictures, but for us, the color was the reason that the idea didn’t materialize. So technically, it is certainly feasible.

Hi,
I don’t see a problem there either. The usual floor construction is a few centimeters of impact sound insulation, then the panels for the underfloor heating, then screed. If you make the impact sound insulation 2cm thinner, you have already compensated for the height difference. Alternatively, you make the impact sound insulation 1cm higher in the rest of the house and 1cm thinner at that spot; this way you can easily compensate for the 2cm 'more' compared to the tiles. No extra effort arises from this, neither for the screed nor for other things.... The screed itself must not be made thinner because otherwise it will break.

Apart from the marble floor, if anything, only the oak beams from the entire timber framing would be interesting, but then rather for furniture making. It was really unbelievable how many carpenters called us when it became known that this Jura house was to be demolished.... Otherwise, the house only contains clay, straw, bricks, and the slate roof.

Just check with dealers for historical building materials. They will surely be happy to tell you what they would still take from you. Are there any old doors, windows, shutters, etc.? They could be interesting for decoration and antique dealers.

Just look for dealers of historic building materials.

To be honest, with the profit margin I was thinking more of myself than of professionals in between. Unless the professional says give it all away right away, no commission, cash on the spot. Then you save on storage and waiting for who wants it when, and have the (smaller) profit in the bag immediately.