Air-to-water heat pump sizing in new construction

That must have nothing to do with clinker. If you have clinker, you will surely have planned it as a double-shell with insulation between the clinker and the interior wall. You achieve the same insulation values as with ETICS.

What kind of walls do you have?

No idea. Ytong, insulation, clinker; inside calcium silicate brick. However, the energy consultant had to put in some effort regarding the thermal bridges so that we achieve the transmission heat loss required for KFW55. Possibly we would have fewer problems with different windows or more/different insulation, but that's how it is now and we are doing 33°C flow temperature.

Heating load is 5.9kW.

Now the focus has to be on proper planning of the heating circuits.

which heat pump was chosen now?

in KFW55

And then it really has to be 33 degrees supply temperature? Unbelievable. I would spend a few euros and have it calculated externally.

Who performed the calculation, the heating engineer or an external party?

Is this supposed to be the room-specific heating load calculation or the calculation of the underfloor heating?

If underfloor heating, what pipe diameter did they use for the calculation, what spacing, and what lengths of the circuits?
How many heating circuits?