For us, the additional costs for underfloor heating were around 8,000 euros.
However, we did not want underfloor heating anyway. With this opinion, we are still "odd birds" in today's time, but we do not regret the decision at all. Our radiators in the house are tiny and do not bother anyone. The climate is pleasant.
Many people do not even notice that we do not have underfloor heating. They are amazed when you tell them and swear up and down that they never saw any radiators....lol.
The financing bank had no problem with it. Even today, not every house is built with underfloor heating, far from it.
As compensation, it was suggested to install a chimney flue. That cost about 4,000. And we went ahead and did that.