We are planning to build in 2022; the planning begins

It says something about 28/35 degrees. But I will clarify that.

35 degrees flow / 28 degrees return
This is the standard 08/15 design for underfloor heating. No thought is given, the standard is simply written down.

35 degrees supply / 28 degrees return This is the run-of-the-mill design for underfloor heating. No thought is given; the standard is simply written down.

And what would be the correct values?

And what would be the correct values?

Values that result from a calculation. So no fixed numbers at all before the most suitable values have been determined. I think you should first find a competent heating engineer, get advice there, and then bring him together with the main contractor. Then he either works as his subcontractor for your construction project or you take heating installation (and possibly air conditioning) out of the "main contractor" scope of work.

Those that resulted from a calculation. So no fixed numbers at all before the most suitable values have been determined.
I think you should first find a competent heating engineer, get advice there, and then bring him together with the general contractor. And then he either takes him as his subcontractor for your construction project or you take heating installation (and possibly air conditioning) out of the "general contractor" package.

And there I know, we agreed that he will calculate it or have it calculated. I assume that he simply inserted a standard number in the offer. I will bring it up again.

I assume that he simply entered a standard number in the offer.

But that is "wrong": elsewhere you also read "according to static requirements," such a formulation belongs here as well. Concrete numbers where "x" belongs are careless.

35/28 means that he lets the heating be calculated based on these values.

This means that the installation spacing is chosen so that the house is warmed to standard values with a flow temperature of 35 degrees.

This is no longer up-to-date today; a underfloor heating suitable for a heat pump should be calculated with 30/28.

In addition, you should specify the temperature for the individual rooms, otherwise 20 degrees is assumed for living areas and 24 degrees for the bathroom.

Only when all this, as well as the heating load for each room, is taken into account can one speak of an individual and adapted calculation.