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

Hmm I think you are right about the matter and still I find you are approaching it too tensely. You are already feeding a gastritis by thinking about contract termination, court, lawyer etc., even though you haven't spoken to the [GU] yet?

I have the feeling that you have found a good [GU], regional, medium-sized, maybe even owner-operated with 1-2 of their own trades? With a bit of luck they also communicate straight – then they will tell you whether it suits them to expand externally or whether they want to be paid for it. And when the options are on the table you can decide.

No worries – they don’t run their business just to mess you around...

Please do not spread slander here! - I was called in too late as the restructuring director of an aluminum window factory. Afterwards, I managed a window dealer who was sold. My successor on the board made mistakes in his registration and led the company into bankruptcy during Corona, and now I am falsely listed as the supposed last CEO in the commercial register. That's not nice – but not a free pass for you to top it off with nasty defamation! Consulting with satisfied customers since 1992, you are welcome to try to imitate that first.

Oh, I didn’t know about the CEO thing – I had referred it to something else. But interesting what you can learn here. You definitely disclose a lot of information, even from before Corona.

As I also wrote to you in the PM: Just leave it. Your condescending opinion is neither law nor the only right way. Your often content-empty, arrogant writing style certainly does not contribute to constructive discussion.

From time to time you do give really useful tips – no question.
But the OP has a problem, and you present it as if it were his own fault. That is neither fair nor helpful, and he even provided the contracts.

I am just wondering here how the OP is supposed to deal with the GC when the detailed planning for stairs, ventilation, water, and similar will result in a reduction of the square meter count because a corresponding shaft or an expansion of the stair opening has to be planned.

Things are being pretty exaggerated here, whereas the essential recommendation is "talk to each other without aggression" and not start thinking about lawyers and who knows what already.

Like , I also rather have the feeling that a good GC was actually caught and such a relationship should not be shattered right away.

(...)
Assuming a lack of motivation from afar is a flat-out accusation. It could be laziness or greed for money or something explainable.

The customer can and should not care about that.
I say this with over 15 years of experience as a supplier of special equipment.

You have sold something and must deliver it, whether you are currently suffering from depression, alcoholism, have no staff, no capacity, or simply no motivation.

The customer is not interested in that. Nor do they have to be.

The customer can and must not care about that.
I say this with well over 15 years of experience as a supplier of special equipment.
You have sold something and must deliver it, regardless of whether you are currently suffering from depression, alcoholism, have no staff, no capacities, or simply no desire.
The customer is not interested in that. Nor do they have to be.

I did not mean that the original poster should accept a mistake or deficiency or the like from the general contractor to his disadvantage; of course not. However, there are many things in such a negotiation beforehand, which is unknown to us, and one should know both sides better to be able to assess them.
Currently, many assume it is a deliberate maneuver by the general contractor to deceive the original poster. If I come to that conclusion, I at least break off contact. But then what? I had asked the original poster about that and other things, there was no answer; from that or from my previous post, I could just as well conclude that the original poster does not take comprehensive and concrete communication quite so seriously or always only wants to see his own side. Exactly such things often happen in conversations, where everyone believes they have said everything, and yet each side has heard something different.
I think that is exactly what you have repeatedly experienced in your 15 years.
The core question for me is not who is right here, but what one does, even if one is right, because in the end there is supposed to be a house standing. From the quoted text of the general contractor, I also read that apparently constructive thoughts have been made there regarding the layout; something the original poster himself wants less.
A good friend always cites her favorite saying: "The one who talks is right." Everyone can always present their own side as right, so one must always know both sides.
It even starts with the fact that as a long-time lurker, the original poster should have known that, as a layman, he would get into trouble. But just like now, he probably did not want to hear critical voices, which he would have gotten if he had posted his project here.

Unfortunately, I was smarter than everyone else here in the forum (have now been a silent lurker for a while) and thought I could rely on the general contractor's planning. Unfortunately, it is starting to get "funny" right at the beginning

In my opinion, the original poster does not understand that pats on the back and being told he is right will not help him, even if he is right. He could have learned here from the existing knowledge opportunities and how he can approach this situation cleverly and successfully. But he did not want to hear that, but rather be confirmed that he is right:

I do not know whether I am rightly upset or if this is just standard in the construction industry.

Your condescending opinion is neither law nor the only right way. Also, your often content-empty, arrogant writing style does not exactly contribute to a constructive discussion.
Occasionally you do have really useful tips – no question about that.

I find it rather arrogant that I supposedly have only "occasionally" really useful tips or that my writing style would be content-empty. On the contrary: I rarely just state something, but often explain very thoroughly why something is a fact – so no one has to believe the great all-knowing eleventh person blindly but gets a comprehensible explanation, always gladly also understandable for laypeople. For an introductory physics seminar to dispel the myth of sand-lime brick as the only sharp dog against the evil sound, however, the typical layperson usually does not have the necessary background.

I am also often misunderstood or considered unfriendly in real life; that is inevitable in communication across the Asperger ./ Neurotypical language barrier. And on the internet, where the acoustic, body language, mimics, etc., dimensions are simply missing in purely written form, I have to endure correspondingly more misunderstandings. But should I really refrain from sharing my expertise just because in Standard German my Berlin bluntness is taken the wrong way?

But the OP has a problem, and you present it as if he were to blame himself.

The OP has a misunderstanding with his general contractor, which emotionally looks like a problem, nothing more. "Blame" is a big, hard, and wrong word for the contributions by both sides to a misunderstanding. The fact is that there was a change on the way between a drawing representation of the contracted construction object interpreted as an agreement and a preliminary draft of the building application drawings, which the OP as a layperson did not expect and which confused him. He is only "to blame" for his attitude towards the general contractor, feeling "fooled" and filing a consumer protection asylum request here in the forum, which is, however, completely unnecessary in factual terms (as mentioned, in the example of the change of the cross gable, the GC even proved that he is precisely not a cunning customer devourer). I then explained to the OP the technical context in which the conflict exists from a construction-technical point of view and that a draftsman – like the OP himself and also myself as laypersons – is not trained to recognize this complication early. Just as the OP probably did not consciously and maliciously pose an unsolvable task to the GC (build the exterior walls in EH40 and with sand-lime brick and in caliber 425), the GC surely did not rub his hands in delight (oh good, I fill the base area with cheap stones and save on expensive interior finishing through reduced living space, he wants it that way). Rather, the GC fulfilled the customer wishes EH 40 and sand-lime brick, which in the magic triangle leads to sacrificing the third parameter caliber 425, and professionally increased the overall wall thickness by a quantum leap to caliber 490. So far, the OP is indeed in agreement with the GC. Then the GC merely – and here I am convinced any insinuation of malice is misplaced! – chose, with a 50/50 chance, the unexpected / different ("wrong" I consider beside the point here) solution to place the difference on the room side. The OP would have preferred it on the outside. That – and no jot more – is already the whole beard of the prophet here. There is no reason anywhere near to dig up a hatchet. Clarify the misunderstanding, laugh together, and shake hands – all done in a moment.

My expertise and my practice as a moderator (and in serious cases client-party) consultant then motivated me to suggest to the OP that he could also fully defuse the conflict he unwittingly provoked by simply exchanging the structural wall builder without disadvantage (because for him the sand-lime brick at this point, compared to aerated concrete, and especially compared to an unfilled porous brick, I would see this differently, offers no advantage but a massive U-value handicap). However, he does not want that. Apparently, feeling deceived is preferable to recognizing in a solution-oriented manner that no one wishes him harm and that (in my opinion better) way out lies outside the hatchet level. There are enough experts not to have to believe me on this without a second opinion. I will not put on the shoe of an arrogance diagnosis for that.