The living space is approximately 8 m² smaller in the application drawings compared to the draft (GU).

So on the left is the draft that the "GU" drew?

"According to the requirements for KfW40 and the (preliminary) information from our structural engineer, we have increased the exterior walls from the original width of 42.5 cm to now 49 cm, and additionally planned interior walls on the ground floor according to structural engineering with b = 17.5 cm. This leads to a slight loss of actual living space, as well as small shifts in the interior walls according to structural engineering. However, this is absolutely necessary.

We have – free of charge – increased the width of the captain’s gable from the original width of 3.49 m to now b = 3.74 m. The reason for this is that the two supports on the ground floor must be about 55 cm wide (structurally required). However, in order not to make the entrance area of the house narrow and small, while at the same time the supports are very massive, and to visually emphasize the supports nicely, we decided to enlarge them."

So what I actually wanted to get at here: Should I now open a can of worms with the general contractor or is this "just the way it is" and something I have to live with now?

Well, then everything is clear!

Clearly no one trying to take advantage of you. By the way, your general contractor plans precisely using the octameter grid, so without cutting bricks / botched pockets. For that reason alone, many builders will envy you – at least those who go to a general contractor unprotected.

As already said, there is no issue to make a fuss about and it is not a case of "that's just how it is." You got an exemplary general contractor and your oversight, not telling him to make a wall thickness adjustment that does not affect the living area (which means then outward), can still be fixed with a short three-line email. That costs him a few mouse clicks, then you have your house 13 cm wider and deeper and the price for it.

Ok, so for me as a layman, this means that this procedure is usual and correct from both a legal and technical point of view?

And if the general contractor then says that I have to pay for the widening of the area on the surfaces as in the design, is that absolutely understandable and comprehensible from your point of view?

It is more than correct; it is actually exemplary!
As I said, the fault lies 1. with you, and 2. can be fixed by a simple little email from your side.

No one other than you has invoked the conflict (EH 40, caliber 425, structural masonry shell made of sand-lime brick – so three requirements, of which only two can always be fulfilled simultaneously), and therefore it is solely in your hands to resolve it. Whether you choose to place the wall thickness increase on the outside or reasonably step back from sand-lime brick for the structural masonry shell is your choice. With aerated concrete, you can revert to caliber 425 here (or possibly to caliber 440, but then you better take the one and a half centimeters from the inside to remain within the external construction dimensions).