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

It wasn't difficult for us, we just built without subsidies and the house is as we wanted.

Yes, that is absolutely the right approach. After all, you only build once :) My comments were not really meant seriously either. Although you can really consider an annex apartment. It increases the resale value and there are more potential buyers who might not be able to afford a large single-family house alone given the rising property prices. But I suspect, as 11ant wrote, that the subsidy for the annex apartment will not be sufficient!?

One only builds once after all

The sentence is often used to steer decisions in a more expensive or supposedly better direction. However, it is completely meaningless and not even universally valid. It is not worth basing decisions on it.

For a new building, it is quite 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. Accordingly, we have placed our bedroom with an adjoining bathroom on the ground floor, 3 children's rooms on the upper floor + a separate complete bathroom with a centrally located family room of 46m² with the connections for the kitchen. The stairwell can be separated towards the ground floor, which is beneficial for heating operation anyway. But we would have built it this way even without the two KFW loans.

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

We separated the children right away (17 and 19 at completion) - they have their own apartments.
Of course, there are other sensible options - such as moving if the house has become too big, keeping the space for a retiree hobby, hosting exchange students, renting out on AirBnB... - Everyone is different.

It is certainly advisable to build in such a way during new construction that separation is possible after the children move out.

That depends on the expected time of the offspring leaving the nest.

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.

Nonsense. I then sell the house that no longer fits and treat myself to something smaller—maybe even suitable for old age as a bungalow.