Supply temperature from 40 degrees to 35 degrees "pattern" or not

I still find 35 degrees too high. For a new building, I would never go above 30 degrees.

This is probably only rarely possible with kfw55. Almost always only with wall and/or electric auxiliary heating.

I made the following specifications: max. supply temperature 30°C with NAT max. 10cm installation spacing specified. max. 20°C room temperature in all rooms. And the smallest possible heat pump suitable for the heating load (otherwise I would have a 5 kW instead of 3.5 kW air-water heat pump).

Conclusion: even the 3.5 kW still cycles in a KFW-55 house at temperatures above 0°C-5°C.

The general contractor also wanted an extra charge for the smaller installation spacing (compared to 15 cm VA) of €1500, which I reduced to €500 by telling him that the smaller heat pump costs him less to purchase, so the stuff was offset against each other and I reluctantly got away cheaper.

Just find out (Google) what the next smaller Daikin system is in terms of performance. And just compare the prices from the internet, maybe then you have a basis.

But in general, VA 40 is already outrageous and cheeky and by no means state of the art for heat pumps.

Important with inverter air-water heat pumps is to look at the big jumps/performance sizes whether both do not modulate down equally anyway, often they are almost identical and only restricted by software.

For example, a 4 kW machine that delivers a minimum of 2 kW performance at 0°C and the 8 kW delivers 56 kW.

And always keep an eye on the min. and max. volume flows of the heat pump compared to what the underfloor heating calculation says. It’s no use if the heat pump delivers 800 l/h at full throttle, but your underfloor heating expects 1500 l/h.

Connection line to the heating circuit distributors at least 28 mm copper pipe and no 25 mm plastic pipe straw with an inner diameter of 20 mm, you won’t be happy with that. I almost assume that from the last time, if the general contractor already wants to cut the price, he certainly won’t lay copper.

I won’t say anything about the bypass valve and buffer, that has been discussed enough here and in other forums. But hey, maybe he’ll gladly leave them out to save money, suggest to him a smaller heat pump then also no bypass valve and buffer (preferably also no individual room control) and instead more underfloor heating pipe.

max. 20°C room temperature in all rooms. Conclusion even the 3.5 kW still cycles in a KFW-55 house at temperatures above 0°C-5°C

Not everyone is sitting in the "fridge". What exactly does cycling mean to you? At 0 degrees, in the conventional sense, certainly nothing cycles. And is your system actually running "properly" now…

max. 20°C room temperature in all rooms.

You mean at least 20 degrees, right?

Yes, it runs properly, lowest heating curve everywhere 21-22 °C [Überströmventil] is removed.

Calculated at max. 20° does not mean that you cannot achieve more later on.

Either through higher flow temperature, electric radiators, or because the nonsense heating load is still oversized even with calculated room temperatures of 20°C.

Strange attitude of your house construction company. Ours insisted on a floor heating system with a closer pipe spacing when choosing the heat pump package. You could practically not buy that separately. Because with such a high supply temperature, your heating costs increase enormously. Heat pumps (at least air-water heat pumps, I don't know others) operate most efficiently at low supply temperatures. Ours is almost always below 30°C even in winter.