GU demands a price increase on the entire house

Well, the main problem with such a process is that the general contractor (GU) can drag it out to the final instance and keep delaying it through tricks. This can take years. The GUs are not stupid; they know exactly that clients have a double burden, which they cannot sustain financially or mentally forever.

That makes it even more difficult. Even without the GU's tricks, such processes tend to drag on for years. Every instance does what it can to get the best outcome for the client. Years of stress come in addition to the financial risk. But I see the financial risk as real and not to be underestimated. Because the longer such a process goes on, the more expensive the whole thing becomes. And I can’t imagine that in this specific case the decision will be 100% in favor of the clients. The wording in the contract is clear in that the lump sum price is time-limited, and unclear as to what should happen afterward. At least from the passages we have seen. And even if the GU does not get what he wants at the end of such a process – something would probably be awarded to him, and the legal costs will almost certainly stick to both parties. A GU can probably handle that more easily than a client.

Oh... I completely forgot. With the last price adjustment at the end of 2021, the wording was also added, roughly "the fixed price now applies until the end of the construction period."

Maybe make sure once again that the general contractor puts this down in writing, otherwise you might have the same hassle again in a year. Who knows how long the construction will take.

As long as the general contractor is doing reasonably well financially, he can easily handle a few clients suing him.
Our electrician once mentioned how many houses of our general contractor he currently has on his list... it was around 100.

Roof covering was a 4 out of 10 for me.

Roof covering is not a deduction for me at all. Normally, that would come directly after the roof truss (for which there is a deduction), then the crane can be dismantled. It's just different everywhere. I also see that a general contractor can hardly get increases approved in court for work that was carried out approximately (we are talking about 1-3 months here) at the scheduled time and has already been done. The price guarantee is also hardly feasible within the usual lead times. I would still avoid letting it come to a lawsuit if possible. It costs a lot of money for lawyers, and you will only know what the house actually costs in a few years. Until then, you can hold tens of thousands of euros and sleep restlessly.

The payment schedule is tied exactly to the completion of certain services such as the roof, shell construction, etc. The costs are incurred by the general contractor predominantly at the time of performance and not only at a later acceptance. Otherwise, the agreed price lock would make no sense at all and could in my opinion be considered unfair or a violation of good faith.

I find it difficult to pay a price adjustment for installed trades retroactively.

The payment schedule is tied to these specific completions of individual sections, trades, or services only as a fallback because these points in time cannot be precisely forecasted as fixed dates, but the installment payment due dates require a specific trigger. Good faith is merely the basis for the effort to reasonably quantify the partial amounts according to the customary values of the contractual parts fulfilled up to that point. The ideal here is a compensation which, although naturally (still) cannot be determined precisely, should not favor either side. A partial acceptance - even fictitious - is not connected with this; accordingly, the view that one is explicitly settled with regard to the partial fulfillments named here is incorrect. The payment tranches do not acquire the partially completed work to date precisely because this was commissioned only as a whole – this would require an explicit termination clause for the construction contract at the designated installment payment dates. Considering the classic general contractor single-family home contract to be a smart move from the client’s point of view can only occur to ardent non-businesspeople.
Summary of this section in "simple language":
1. The installment payments up to point x DO NOT pay for the construction status achieved until then.
2. The construction contract is also NOT simply terminable (and compensable) at this point;
the belief that one then owes each other nothing anymore is legally wrong.

The agreed price lock of an offer fits best to reality if it extends until the completion and acceptance of the work. However, it does not automatically become unfair simply because it is agreed for a shorter period. A specification of events that delay or suspend the deadline is sensible but not mandatory. Building clients cannot be advised often enough not to carry out legal transactions in the dimension of a construction contract without legally knowledgeable advice. I am a technical businessman with many years of experience and would still never have my house built without consulting a lawyer. From trustworthy sources I have so far unanimously heard that the "Builder’s Driver’s License" by lawyer Reibold-Rolinger is very highly recommended.

But even if you say you have to look at the entire house, I still wonder. If there is a 12-month price guarantee and the house is finished after 16 months... that doesn’t really sound fair to me if the builder has to pay the entire additional cost that would arise from a new contract today. He is already calculating a buffer for what happens during the construction time, and the remaining construction time here is significantly shorter. Moreover, I had hoped that one might pay the increases for the 4-month delay, but not the entire increase for the total time.

I assume (completely unfounded, just guessing) that a real legal claim looks difficult there. It would perhaps also be interesting to know how this is structured in other contracts when there is a time-limited guarantee (does not mean you can simply transfer it, as they are designed differently. But it might give you ideas for argumentation?).

The "buffer" that the general contractor (GU) calculates is called the profit margin. Basically, everything here has already been said multiple times?

The deductions in the contract do NOT represent partial acceptance of individual trades. Strictly speaking, the enclosed shell is not even completed yet, as the roof covering is missing.

And even if one were partially right. Enforcing these claims will be many times more expensive than agreeing with the GU on an amount x.

The lawyer, who would make a good profit from such a legal dispute, advises against it. What else is needed to realize that it is better to simply stop?