Whom to entrust with the room-wise heating load calculation?

You are mixing things up. It’s not about the sizing of the heat pump, but about the design of the underfloor heating. For me, the calculation took two evenings. More time is wasted on less useful things during house construction. But now I can show the HB that 30/25, for example, works well.

It’s about achieving an efficient heating system. That starts with the sizing of the heat generator and continues with the design of the underfloor heating. Why do you want to show your HB anything? For what purpose, what’s the use? Lay out 5-10 cm spacing and as much surface area as possible. Then you can play with the heating curve however you like. That’s exactly what I mean – the calculation has no practical impact on implementation except the insight that you have to lay it densely and with a lot of coverage, and even the smallest heat pump is sufficient. This applies across the board to all single-family homes up to 200 sqm with controlled residential ventilation, I claim. It’s that simple.

Don’t get me wrong, I also forced my HB to do a heat load calculation because otherwise he would have oversized the heat pump. And he didn’t want to be responsible for going smaller. Proof was necessary there. He did lay the underfloor heating closely anyway; in the bathroom I pressed for 5 cm spacing and to include the shower. It was simply a cost issue; the pipe has to be paid for, but he doesn’t resist that.

And 10 cm is enough in normal rooms. The only issue with it being too cold could be in the bathroom if significantly higher temperatures are desired there.

It's about having an efficient heating system. That starts with the sizing of the heat generator and extends to the design of the underfloor heating.
Why do you want to show your HB something? For what purpose, what does it bring?

I agree with you, it's about an efficient heating system.

Why do I want to show something? Because not every heating installer is so flexible to accommodate all customer wishes. Sometimes you hear "that’s not possible." I feel better if I have something in my hands.
Yes, as I wrote above, if you are satisfied with supply/return 35/30 then "your system" is sufficient. If you go lower, it depends a lot on your insulation standard and the rooms. For me, the children's rooms upstairs only work at 30/25 with additional surface area on the wall. (high ceilings, lots of window area).
In some hallway areas, in the basement, etc., I don’t need 10cm; 10cm would be way over the top there. Why should I put that much piping in there then?
Could one have guessed that? No idea.
As I said, it cost me two evenings to find that out.

As already written before. To get warm, the exact design is certainly not necessary. But if I want permanently low heating costs, I have to optimize the overall system in interaction with the following parameters:

Supply/return temperatures
Flow rate
Heating capacity

If I then save 100-200 € heating costs per year, why not?

The optimization and adjustment of the heat pumps is currently more of the problem. No one pays attention to that, and for the most part, not even the heating engineer deals with it, and as is often read here in the forum and in the Rosanen, the bathroom is almost always the problem. Even with us, about 1.5 degrees were missing because an additional heater was calculated, which Schwörerhaus messed up, and at that time we were not aware of the real impact it has. I also didn't pay much attention to it in the first years. After a defect of the circuit board in December, but thanks to the Rosa forum, I think I got pretty good values. Before that, there were 5000-6000 kWh per year. Daily consumption around 25 kWh at kfw70 and about 240 m2 heating area. If you assume that January accounts for 25% of the annual output, we come to about 4000 kWh. And that is good for us.

Anyone who needs an address for a heating load calculation, for us it was the company Schuetz, which also sells distribution boards and switches, etc.

So I also had my underfloor heating design done by an engineering office that I found through Ebay Classifieds. It cost me 180 € for the room-specific heating load and I was satisfied with the quality.

Our energy consultant offers us a heating load calculation. However, I do not yet know how detailed it will be. We found him through the [KfW] website. However, we are not building, but renovating. I am curious how the heating will work for us. We are using a floor heating retrofit system, which only has 5cm and 10cm installation spacing, nothing in between.