GU demands a price increase on the entire house

Even though it is of course annoying for everyone. This is not fraud but simply the current market situation. The general contractor (GU) also has to purchase at the increased prices. And even if you refer to your contract. What if the GU simply goes into insolvency? Then you have a house in shell condition, which will presumably be completed by another GU at 30-40% higher costs or you tender the trades yourselves and also end up with those amounts. Better to agree now on 20-25k and carry it through. It is of no use to you now to consider who is legally on the right side, since you want a finished house before there is a wave of insolvencies to be feared in the construction industry, because the high prices not only eat up every margin but also cause losses.

Since we already have a longer history of suffering... Signed at the end of 2019, hoping for a quick development of the building area.

Oh. Signing the contract without a finished building plot was certainly not the smartest idea.

2021 was really the worst year to start building... or in our case, not to start on time...

For you personally, maybe. I wouldn't want to be in the shoes of the builders who had to decide/must have decided this year.

Sure. Just tell the general contractor, "Hey, I'll do the rest myself, you've got enough money anyway, I think."

Well, before the money is gone and the roof also gets damaged, I would rather choose the lesser evil. If you're already suing, you might as well include that too.

An acceptance of the trades or of the entire contract plays absolutely no role in the creation of additional costs and in my opinion has no influence at all on the enforcement of higher costs. The question is whether exceeding the deadline of the flat rate price can automatically lead to an arbitrary readjustment of the price? [...] One must be allowed to assume that the price was cost-covering for the general contractor until day X. From day X onwards, no longer. Accordingly, he has to break down the costs.

That the OP is mistaken in considering the achieved construction status as "checked off" is already significant. However, I agree with you that the general contractor should nevertheless treat this, let’s say, "past" as a fictitious "reading date" if one wants to act properly by mutual agreement. Legally, there will rather be a clear assignment of roles as to who the loser is here: namely the one who was more negligent during the drafting of the contract. At least one of the contracting parties probably thought too little about what should apply if the work is not completed within the planned time. If the contractor did not take into account his risk of underestimating cost increases here, then one will have to look to the law as a fallback (here: Building Code). The gap in the contract is of course more favorable to both sides to be settled by voluntary agreement. The law can certainly also be applied without a judge’s ruling ;-)

So, generally, if I buy a house and the contract states a fixed price with the associated services that have to be provided for this price, then as a buyer I would assume that the materials for this house are already paid for and in stock at that point in time.

At least you may assume that price increases are not your problem. Based on the little we know here about the individual case, my assessment is that both sides were probably negligent when drafting the contract. Accordingly, I would suspect a court ruling close to the "mediation result," voluntarily applying the law.

Late reply from me. It was a bit stressful today… an interesting discussion has also developed here. I like it. So the result of the phone call was not really satisfactory. He explained his point of view (which I also understand) and I explained ours… played a little on the sympathy angle. The result is that he will take another look at the numbers next week and I think we will settle somewhere between 15-25k. It's annoying but not a disaster. We both don’t want a legal dispute that just becomes unnecessarily expensive for both sides. We can finally say about ourselves that we were dumb, young, and naive when signing the contract. We have to admit that mistake. I was also too optimistic regarding the 12 months. I thought everything would go faster. But authorities and the civil engineer threw a wrench into that. Learned from it in case I build again :D I’m considering the additional payment as tuition paid for life. Thanks to everyone for the helpful tips and the nice discussion. I actually had a bit of concern before posting. But you’re quite nice after all :D

That changes significantly after the contract is concluded.
Price changes in existing contracts regularly lead to special termination rights.

Because the price fixity expires without defining what happens then, and the contract also does not define that the price does not change from the start of construction or that the trades are accepted and invoiced individually.

You are on the wrong track. Morally you may be right, but legally not. He ended up with an unfair general contractor, but that doesn't help now.