How does one correctly balance between the Building Energy Act and KfW 55, 40, and 40 Plus?

I can probably best judge myself with what compromises I waste how much money.

You can do that. But the majority has nothing to give away ;-) The majority simply wants to live nicely with as manageable costs as possible and as few compromises as possible.

The fact is, tens of millions of square meters of living space in Germany are empty precisely because of this situation. That is economic nonsense.

That is only partly true. Vacancy yes, but for other reasons. A good example is the Ruhr area. High vacancy and yet high housing shortage. Vacancy is garbage, reasonable (and sought-after) housing is scarce. And in rural areas there is enough vacancy. Nobody wants to go there anymore. I am talking about dying villages without offspring, often in the East. This is also reflected in the prices.

You can do that. But the majority has nothing to give away ;-) The majority simply wants to live nicely with as manageable costs as possible and as few compromises as possible. That is only partly true. Vacancy yes, but for other reasons. A good example is the Ruhr area. High vacancy and yet high housing shortage. Vacancy is rubbish, sensible (and sought-after) housing is scarce. And in the countryside there is enough vacancy. Nobody wants to go there anymore. I am talking about dying villages without offspring, preferably in the East. This is also reflected in the prices.

That fits well. I live in the countryside. And in a community of 2000 inhabitants we have already reached 250 €/m2 for building land here. Families with three children do not get land because it is rationed by lottery. Reality 2021, thanks to the euro preservation policy.

When building a new house, it is certainly advisable to build in such a way that it can be separated after the children move out.

With your described 3 children's rooms + large bathroom + extra room, this is of course recommended, otherwise it is hopelessly too large. With about 150m², you might have a guest room (for children/grandchildren visiting?) and a hobby room. At least that’s how I estimate it.

When building a new house, it is definitely advisable to build in such a way that it can be separated after the children move out. Either one of the children occupies the separate apartment or it is rented out externally. Those who can afford it in old age can still leave it empty. However, that is not ecological.

It depends on how big you build and how the house is. I can see it with my parents right now, with 120sqm there's not much room for separation and the former children's rooms upstairs are currently used as an office and guest room. Selling and moving into an apartment is no option in the medium term, the ground floor is now barrier-free and you don't live as nicely with a garden in any apartment anyway. As long as you are still fit, that works well and afterwards it hardly makes any difference. I had also considered the issue of separation, but that forces a staircase directly at the entrance and thus ruins most floor plans. Suddenly you have unnecessarily long corridors upstairs, a bathroom with a lot of sloping ceilings, etc., and downstairs it doesn't fit either because the open-plan room becomes significantly smaller if you have more plans. In the end, I decided to build for ourselves first. If necessary, an outside staircase will be installed later, a drywall erected on the ground floor, and the original staircase replaced by an additional ceiling. If that case ever becomes relevant at all.