Why don't construction prices go down?

You describe the theory, I say how it will go in practice. No one will rebuild the wall for you. And if you don't pay the deposit according to the contract, the general contractor won't continue building. […] Mistakes can always happen. Presumably someone sawed the first layer incorrectly, reading error, wrong section taken, something like that... and then it was just built on.

No one said in theory that the wall would be rebuilt in practice. That's just your theory-averse part of the brain imagining things ;-) . If it was sawed, I see it the same way: a mistake happened in the first layer, which then propagates. It is still a defect of this magnitude, even for a rough trade like walls. I would now check if and, if so, how much living space is lost. Then possibly report the defect at the last payment installment. As late as possible. Inform the kitchen studio as early as possible and discuss alternatives.

That's why the question about the other wall. If it is shifted and the room is larger on the other side, then I agree with you. If the 9 cm are simply missing, I would assume more. If the whole house is smaller because of that, it can already be over 1 m² and then that's a few thousand EUR.

Ultimately, a cabinet has to be changed from 90 to 80 or from 60 to 50. That's all there is to it.

Exactly - with 4 identical cabinets planned, it just looks silly... And then somewhere there is still a 7 cm strip...

Inform the kitchen studio as early as possible and discuss alternatives.

They were there for the measurement and noticed that...

We haven't measured anything else...

Rough construction dimension: 407.5 Planning dimension kitchen: 4.01 Actual room dimension: 3.98

But the room dimension is with plaster already, right? Or still rough construction?

Is the room measurement now taken with plaster already? Or still raw construction?

With plaster....

Planner + bricklayer same company...
Plasterer subcontractor of the general contractor...

Go back to the kitchen fitter and think about how to make it coherent. The panels can be adapted to 2x3.5cm, so it won't stand out as much. At the time, we also came from the appointment with the kitchen fitter to the construction site and had to realize that in the freshly completed ground floor with the intermediate ceiling poured in the morning, the dimension of 10cm was missing. In the end, it turned out that the plan had been changed because the structural engineer wanted more load-bearing walls...