GU demands a price increase on the entire house

Word of mouth

What kind of outrage is this? O_O

What kind of mess is that? O_O

That's what we say here - completely suitable for all ages. Different countries, different m... uh, customs.

That's what we say here - completely family-friendly. Different countries, different t... uh customs.

I would have accepted word-of-mouth ;-)
Here we call it word-of-mouth advertising.

In the end, advertising has to work.

So generally, when I buy a house and the contract states a fixed price with the corresponding services that must be provided for that price,
then as the buyer, I assume that the material for this house is already paid for and in stock at that time.

I would not assume that at all. Building material costs a fortune.
My general contractor (GC) told me recently that he can only offer fixed lump sums and keep them because he has the necessary liquidity to advance payments. He already had bricks in the back of his pickup before our foundation slab was even poured and asked us if we had finalized the color and if it was okay, because he wanted to order them. The heating system was already stored on site.
But that is anything but usual. He said that until the drastic price increases, he usually purchased his materials "on the fly," directly from the building materials dealer to the construction site. Meanwhile, he needs a lot more storage space.
Not every company has both the storage space and the liquidity to advance all costs.

...
So, the result of the phone call was not really satisfactory...
The result is that he will review the numbers again next week and I think we will agree somewhere around 15-25k.

I think, considering your situation, you could be very satisfied with that. You always have to consider, what would the alternatives be if you don’t reach an amicable agreement?
In my opinion, the child has already fallen into the well when the contract was signed. You say that yourself. Now you are where you are, and in this situation, an agreement on the amount and an amicable parting are probably the cheapest option and also the one that costs you the least nerves. One should also not underestimate what years of disputes / legal battles do to one’s psyche and health. It is a lot of money, but better an end with a fright than a fright without end.

I can’t believe it, 25k more without disclosure just like that because everything got more expensive. Nice that the margin was saved...

You can believe it or not, but the signed contract (at least what we have seen of it) is worded disadvantageously for the builder. And it was accepted that way. Paper is patient, and legally you don’t have a particularly good foundation to argue with the GC.

Is a big GC. Sorry, at the beginning I confused GC and general contractor (GU).

Don’t worry about it, I’ve been here for years and still can’t tell the difference.

Of course, it would also have been interesting to know at what price level you were operating before the increase? It cannot be ruled out that the GU did not raise prices as sharply as others in 2021 but is now making up for it in 2022. This can also have strategic reasons. Who knows?

He said that up until the huge increases, he also bought his material rather "on the fly," directly from the building materials supplier to the construction site. Meanwhile, he needs much more storage space. Not all companies have both the storage space and the liquidity to advance everything.

That he acts this way and can afford it without feeling like the fool already speaks in an almost incredible way for him as a recommendable entrepreneur and accordingly for your lucky choice. However, own liquidity also requires capital service (then called return). The business of general contractors has become more uncomfortable ...

and in this situation, agreeing on the amount and an amicable separation is probably the most favorable option,

... but that is the entrepreneurial risk of the contractor, and is legally and morally none of the client's business. Nevertheless, there may be reasons – also selfish ones – for the client to listen not only to his commercial but also to his Christian heart. However: if I pay a voluntary price surcharge as a customer in such a situation, it has two dimensions. One is an investment in keeping my warranty opponent capable of acting – this can prove to be a sham investment aka unfortunate speculation. He can still go insolvent during the warranty period, and then I as a customer am, politely put, not one of those with the best prospect of satisfaction. Then I am left with only the second dimension, to enjoy my blessed Christian heart (and to admit that this goodwill is a luxury from a commercial point of view). Because the money from the price surcharge is gone, and it has at no time increased the value of the completed work – neither for me nor as security for the financing bank. Personally, I would provide such a private safety net for my contractor exclusively if I thereby secured the completion of my house and effectively protected its commercial value for myself. Exactly this, however, is only "pseudo-necessary" here due to the poster’s negligence in contract drafting. The inclined reader should learn from this that suitable points of the payment plan (e.g., the construction status "weatherproof shell") or in a way subsequent price changes should also be made possible termination points in contract design. If the poster had considered this, he could now calmly retreat to the standpoint along the lines of WYSIWYG, consider the achieved construction status as "his," and allocate the continuation of the further construction at free choice without having to fear protest from the general contractor.