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

The difference is now a loss of 9.57 m².

So you now have 1.57 m² more right, you don't give up. Nobody doubts that you should build your desired living space or that there are either unclean dealings, a misunderstanding, or something else going on here, whether it’s 8 or 18 m²... the question is how you find out that quickly and as best as possible, you also question yourself again for safety’s sake and – here it comes – what you do afterwards. From you I read "opening a case," legal action, etc., so apparently you believe you can scare someone with that and overlook that you ultimately only harm yourself and with the highest probability then do not get “your” right, because such courts are known to seek compromises. How can one waste energy like that, the lawyer will get more work out of it. The at least thought-provoking hint that you might have already senselessly given away some equally expensive square meters yourself you overlook or ignore, which then surprises me in view of your statements about 15 years of saving and the loss calculation per m². So if the general contractor now admits his meanness and gives you back the 9.x m² AND you also correct your possibly gifted away m² you would have saved twice as much money. But then again it’s not so bad if €20,000 maybe get wasted uselessly. So it can’t be about the money – it seems to me it’s more about being right. I am curious whether you will share your result with the general contractor here just as openly. It remains exciting.

Unfortunately, you misunderstood me. The order before signing the contract was a KfW 40 single-family house made of 17.5 cm sand-lime brick + insulation (no dimensional specifications given here) + 11.5 cm facing brick. I did not know how thick the insulation had to be to achieve KfW 40. I left that to the general contractor. Subsequently, the contractor created the colorful drawings you have before you. I trusted that these drawings already corresponded to the wall construction according to KfW 40 requirements and that we could rely on the stated areas. I never wrote that the entire wall construction in its width should be a 425 caliber. After signing, the contractor sent me the information along with the building application drawings that previously a 425-caliber wall construction had been calculated, which unfortunately has now been widened by 6.5 cm: [ / ]
As a specification, as already mentioned, I stated sand-lime brick 17.5 + insulation + 11.5 facing brick to the general contractor at first contact. No further details regarding the thickness of the insulation and especially not the total wall thickness. I also requested that the whole be executed as eh40. I had trusted that the sketches created already complied with the eh40 requirements and that we could be happy about the stated areas. I never mentioned a 425 caliber. How am I supposed to know as a layman how thick the insulation has to be? I then placed the order.

I did not misunderstand you. You did not explicitly request "caliber 425," that is correct. But you expected a total wall thickness as shown in the colorful drawings, even though the necessary architectural services had not yet been completed and the colorful drawings were still target agreements to be checked for consistency. Perhaps the general contractor should improve customer clarification in this respect. Personally, I also consider it better to secure the general contractor’s effort in preparing the plans through the “deposit solution” (planning services paid in advance and offset in case of order) rather than with the method "final drawings only after contract signing." Of course, it does not have to be apparent to both laymen (= you as well as the draftsman or customer advisor) that the standard wall thickness probably works with EH40, but not with the contradiction of EH40 and sand-lime brick.

A short time later, I was sent the building application drawings and the further information already mentioned:
"According to the requirements for KfW40 and the (preliminary) statements of our structural engineer, we have expanded the exterior walls from the original width = 42.5 cm to now 49 cm; additionally, interior walls on the ground floor have been planned with b = 17.5 cm according to statics, which leads to a small loss of actual living space, as well as small shifts of interior walls according to statics. However, this is absolutely necessary."

The wording that the wall thickness of 49 cm is “absolutely necessary period” is awkwardly chosen. It should properly say that it is “absolutely necessary comma if you insist on sand-lime brick for the structural masonry shell.” One could have sought dialogue with you here and advised you that there are several ways out, namely:
1. Increase the insulation thickness to compensate for the chosen material sand-lime brick, at the expense of a non-system-compliant total wall thickness;
2. Increase the insulation thickness to compensate for the chosen material sand-lime brick, with an increase of the total wall thickness by a full quantum leap to 49 cm;
3. Replace the chosen material sand-lime brick with aerated concrete to maintain the wall thickness as drawn.

I wrote how I should proceed now. Unfortunately, I don’t know any other way to handle this, I have to be honest about that. And yes, of course, I will first approach the matter objectively and simply ask to move the walls outward so that the contractually owed room areas at least reasonably match again. By the way, I just recalculated now, since I’m finally free with a somewhat clearer head.
The originally planned living area was 164.84 m² according to DIN.
The new living area is 155.27 m² according to DIN.
The difference is now a loss of 9.57 m².

I already said how you can easily help yourself, namely with a simple email:
"I am surprised by the total wall thickness stated as necessary and consider the room sizes previously communicated to me as agreed. Please shift the wall thickness increase to the outside, even if the exterior dimensions of the house – including height – thereby increase."

They were harmless communication deficiencies. They became a “problem” only in your mind.
De-demonize them by treating them again as communication deficiencies.

The difference is now a loss of 9.57 m².

Don't drive yourself crazy! I see it exactly the same way as you do. However, it doesn't make any sense to immediately wield the lawyer's club. First, talk. Present your point of view, discuss, and probably renegotiate in the end. The money you would have to give the lawyer is better kept in mind as bargaining power. Besides, you should also consider whether you'd really be better off elsewhere. And always sleep on it first in peace. Like now... Good night!

I find it quite far-fetched that a court would seek a compromise here. Because nothing has happened yet that would represent an unreasonable effort for the contractor to fix.

Compromises are made when the shell is already standing and you can’t change anything without tearing everything down again. Compromises are made when the screed is laid and a radiator was forgotten. Things like that.

Apart from that, I would never even consider a lawsuit against a contractor at this stage. Talk to the contractor and ask them to keep the living area. If that doesn’t work, then you have to find another one. Terminate the contract and see if the contractor is confident enough in being right to claim compensation for his "services rendered." I rather doubt it.

Talk to the builder and ask them to maintain the living area. If that doesn't work, then you have to find another one. Then cancel the contract and see if the builder feels confident enough in being right to sue for compensation for their "services rendered."

Well then we finally agree (with) (m)e on opinion, that was exactly my suggestion and not just mine. Clearly talk to each other, then just as clearly decide.

I consider it quite far-fetched that a court would seek a compromise here. Because nothing has happened here yet that would constitute an unreasonable effort for the builder to fix.

Since you like to boast about your professional experience in a specific field, you should also recognize where you lack it. Civil lawsuits end in settlements to a very high degree, rarely does someone get the desired 100% right, and not because the facts would be unclear, but often for purely economic reasons. Also for this reason, a reference to the legal route is often a shot, or at least a stray shot, in one’s own knee. I wanted to warn the OP against this "threatening gesture" which may seem potentially effective.

I rather suspect not.

I had also advised the OP against assumptions and hopes and recommended the open, clarifying conversation, just as you explained at the beginning.

By the way, energy standards or the proof of their compliance can be calculated in two ways. Either as a flat rate, then each component (roof, exterior walls, windows, floor) must meet a certain U-value limit. This is probably exceeded here because of [KSS]. But there is also the exact overall calculation. Then the transmission heat loss of the entire building is calculated room by room, and thus the lower insulation of one component can be compensated by the better insulation of another. That is more expensive (both the proof and most likely the implementation), but it would still be an option to keep the wall thickness as original. In addition, one could of course improve the insulation material. That also gets more expensive, but hey, if it’s important to someone, vacuum panels can also be installed (not entirely serious). Regarding the matter, I recommend what many others have suggested here: ask about the possibility of retaining the net floor area. For that, the general contractor should be able to offer several options with different consequences.