Is underfloor heating in the basement useful??

I just don't understand now why underfloor heating with individual room control is still installed nowadays when it is actually nonsense? We have a KFW55 house, the house will definitely be well insulated.

Lobbyism, economic development, conservatism... take your pick. If you want constant temperatures in the house, you should forbid a floor heating system from being "smart." In a properly insulated house, you have two or three degrees more at the floor than in the air. If the temperature in the room then rises (due to the sun, for example), the floor can no longer give off temperature. Then nothing needs to be regulated. Physics takes care of that. (There are plenty of papers on the self-regulating effect).

What definitely needs to be done with your heating is to make the 60° a floor heating-compatible temperature. Ideally, this is done via an appropriate heat exchanger and then directly into the floor heating. Nonsense would be an (additional) buffer storage and preferably also mixing in cold water. But these are just wild guesses, as I don't know anything about this. If you ever switch to a heat pump, all that stuff can be removed again. It goes directly into the heating circuit because it can produce the right temperature itself.

Yes, only 30 degrees will flow through the underfloor heating, I don't know exactly how that works technically.

Would it then make sense for us to replace the individual room controllers with "smart" ones?

...
If you want constant temperatures in the house anyway, you should forbid underfloor heating from being "smart". In a properly insulated house, the floor is two or three degrees warmer than the air. If the temperature in the room then rises (due to the sun, for example), the floor can no longer give off heat. Then nothing needs to be regulated. Physics takes care of it. (There are plenty of treatises on the self-regulating effect)....

...
Would it then make sense for us to replace the individual room controllers with "smart" ones?

HÄ?? ;) (And a few more unnecessary characters)

What options do I have? Our builder will equip the underfloor heating with individual room controllers that will then turn the underfloor heating on and off. How can I prevent that? And how can I then set the heating in our home?

Please ask him to have the individual room controllers removed. With a coherent concept including balancing, this will be approved. If he cannot do that, he is bad. If you have no choice, he should - as described above - install actuators that are open without power, i.e., let all the water through. Also tell him that you will unplug the connectors BEFORE he performs the hydraulic balancing (which he is legally obliged to do). The connectors will then remain unplugged; you can sell the control unit and the motors. How you regulate individual rooms afterwards, he must explain to you upon handover. Furthermore, you want the installer's access to the heating system. If something like that exists at district heating at all. At least, that is how I would proceed. That would be the least work / the least cost for you.