Hello,
The building is without a basement on a slab.
Ok. Basically, it is possible to move the chimney flue; but the effort and the costs are immense, since a floor ceiling as well as the roof covering were penetrated.
I would really be grateful for any solution
Stop drawing - no one with sense arranges rooms around a chimney flue; rather, one dispenses with it - and wait for the conversation with your expert. I could imagine that the most cost-effective solution for your builder is a dismantling of the chimney flue. Instead, an external stainless steel flue and some rethinking on your part about the location where the later chimney will be installed. It is still early enough to omit the underfloor heating and to adjust the floor structure to the later chimney.
Whatever you decide, make sure that any communication with your builder on this topic - involving the expert - is done in writing. Sooner or later, the term "proportionality" will come up, and naturally, builders and the builder differ in their interpretation of it.
Rhenish regards