KfW - construction: is it necessary or not?

On the one hand, the sun also shines in winter, on the other hand, it is a naive calculation. Compared to a gas heating system, green electricity from the socket is still more ecological. With the appropriate insulation, even more so.

I believe that you cannot base it on individual values. Many people drive fuel-efficient cars but unnecessarily cover a lot of kilometers or have the craziest heating systems yet waste an infinite amount of resources through their daily behavior or install ecologically pointless things elsewhere in the house. Heating, water, and electricity consumption with care serves ecology much more than squeezing out the last calculated percent somewhere, which I then multiply throw out the window again in everyday life at the thermostat. I have met people in the world who bury their waste behind the house or throw it into the water. Nevertheless, compared to us here, they live many times more ecologically because they do not even have all that stuff available. Therefore, I believe that one should consider the entire package for oneself.

But you can also try to challenge the dominance of eco-houses by force
Sure - it still depends on the overall package.

I don't want to at all
I just want to point out that not everything labeled "this will make my house an eco-house" really leads to an ecologically less harmful house. A lot sounds good in the brochure, but overall calculated it may even be more harmful/resource-intensive than a "conventional" solution.
On the contrary, I often unfortunately feel that especially builders who construct an "eco-house" are very missionary. Without really taking all components into account.

I believe that it cannot be determined by individual values. Many people drive fuel-efficient cars but unnecessarily many kilometers, or have the craziest heating systems but waste infinite resources through their daily behavior or install ecologically pointless things elsewhere in the house. Heating, water, and electricity consumption with care serves ecology significantly more than squeezing out the last calculative percent somewhere, which I then repeatedly throw out of the window in daily life at the thermostat. I have met people around the world who bury their waste behind the house or throw it into the water. Nevertheless, they live ecologically many times more than we do here because they simply do not have all that stuff available. Therefore, I believe that one should consider the entire package for oneself.

And yet, compared to the heat pump, the gas heating system is not more ecological. I don't care at all how others build their houses, really, I have absolutely no claim to the ultimate, morally correct answer to climate change – and I mean that completely seriously. What I do find a bit hypocritical, however, is to pretend that eco-houses and heat pumps and alternatives are not better anyway than, for example, gas heating, because one also drives a car... That makes no sense. Either I want to build ecologically or not.

Compared to a gas heating system, green electricity from the socket is still more ecological. With appropriate insulation, even more so.

No. Suppose you build your house with a heat pump and source 100% green electricity. Since only a limited amount of this electricity is available, electricity must be generated with coal or gas to compensate for your consumption. This can lead to the situation where it would be more ecological to produce little CO2 with an efficient condensing boiler than to generate electricity with "a lot" of CO2 in a coal power plant.

advised me not to build a KfW house.

That's a typical "pub conversation," just in construction. Ask three people and you get four opinions.

a) you would need a construction supervisor (who is also specialized in that area)

So what? First of all, it depends on how you build. Via an architect? Then he might even be qualified enough himself, or he can arrange something cheaply. Or you build a standard house via a general contractor, then that is priced accordingly. And even if you want to find and hire a consultant yourself, you can pick either an inexpensive or expensive one.

b) if you insulate the house well, etc., the ancillary costs are low.

What an empty statement. When the sun sets, it gets dark....

As said, it's also about maintaining the subsidy, and the consultant will tell you which measures make sense and which do not.

Furthermore, he recommended taking a gas boiler + solar including battery instead of an air-water heat pump + solar including battery.

Nonsense.
A battery is not financially worthwhile. My recommendation: leave it out.
Whether gas or air-water heat pump, in my opinion, doesn't matter. Subsidy-wise, air-water heat pumps are well supported and, combined with underfloor heating, a great thing. On the other hand, I consider gas very mature and technically quite inexpensive. With the new pipelines, the gas price remains low.

Since only a limited amount of this electricity is available, coal or gas must be used to generate electricity to balance your consumption.

This can lead to a situation where it would be more ecological to produce little CO2 with an efficient condensing boiler rather than generating electricity with "a lot" of CO2 in a coal power plant.

If you already draw the balance up to electricity generation, you must not stop before the flow of money. Because if I sign an electricity supply contract with a purely green electricity provider, they in turn invest the money exclusively in renewable electricity generation. Thus, this leads to an expansion of exactly these electricity generation sources, which makes the overall electricity mix more ecological.