KfW - construction: is it necessary or not?

Not necessary. Our house is without KFW classification. It's okay too.

... who get their electricity during the heating season from fossil fuels? Or from the nuclear power plant? The photovoltaic system does not help in winter.

Then the heat pump is ecological?

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 grid is still more ecological. With the appropriate insulation, even more so.

With the appropriate insulation even more so.

... which, however, also has to be produced and later disposed of again (see all the Styrofoam waste that was produced for a long time/still is).

If you really want to calculate it, it becomes a very complex matter. In the end, the uninsulated old building often comes off better - especially if the alternative is demolition, disposal, and resource-intensive new construction. Everything is very relative and depends on the perspective. The ecological badge will certainly be quickly pinned on from all sides.

Maybe it’s also due to age, but sometimes I feel that nowadays way too many so-called standards are being set among the builders themselves, as if you simply cannot build anymore without this or that.
I read, among other things, that a house without underfloor heating is complete nonsense, that all kinds of xy values are constantly being outdone or that only this or that seems possible and is taken as fact.
This also applies often to the topic of Kfw.
In the end, I want to feel really comfortable in my very individually designed place, and more value should be placed on that in detail; but unfortunately, the money often runs out there. I have never or only partially had all that around KFW, controlled residential ventilation, underfloor heating, etc. etc. in my various living situations and yet I have always made sure that I feel great in the house.
I like bold individuality in design, which no one else can feel for me.
With or without KFW – that won’t change anything about your well-being in the house. If it is insignificantly more expensive, you can go for it; otherwise, I would invest the money elsewhere in directly noticeable living and quality of life.

Not necessary. Our house has no KfW classification. Works fine.

And so you are not yet bankrupt because of all the exorbitant additional costs?

... which, of course, also has to be produced and later disposed of again (see all the Styrofoam waste that has been produced for a long time/is still being produced).

If you really want to tally it all up, it becomes a very complex matter.
In the end, often even the uninsulated old building comes out better - if the alternative is demolition, disposal, and a resource-intensive new building. Everything is very relative and dependent on the point of view. The ecological badge, in any case, is gladly and quickly pinned on from all sides.

But we are talking here a) about a new building and b) there are now so many insulating options outside of Styrofoam...
But you can also try by force to question the dominance of eco-houses
Sure - it still depends on the overall package.

Maybe it’s also because of age, but sometimes I have the feeling that nowadays way too many so-called standards are being set among builders themselves, as if you can’t build anymore without this or that nowadays.
I read, among other things, that a house without underfloor heating is complete nonsense, that all sorts of xy-values are mutually outdone or that only one or the other seems possible and is considered fact.
This also often applies to the Kfw topic.
In the end, I want to feel really comfortable in my very individually designed place and more importance should be placed on that in detail; unfortunately, money often runs out there. I have not had all the stuff about KFW, controlled residential ventilation, underfloor heating, etc., in my various living situations so far or only partially, and yet I have always made sure that I feel great in the house.
I like bold individuality in design, which no one else can feel for me.
With or without KFW - that will change nothing about your well-being in the house. If it is insignificantly more expensive you can do it, otherwise I would invest the money elsewhere in directly tangible living and quality of life.

Supposedly there are actually people who feel more comfortable if their house has a good ecological footprint... But maybe that’s just a rumor
There are also people here who claim a house without a basement is not a house. Shows you how different the priorities can be.

And with that, you’re not yet broke because of all those exorbitant additional costs?

Personally, I don’t think it’s about ecology for you, as at least hinted in your question - which is no accusation. As said, I consider that a completely subjective prioritization.

Fact is: You don’t have to build to Kfw standard; additional costs can certainly be minimized in other ways as well.