Fine staking is wrong, who is to blame?

I agree with my predecessors.

Construction projects are simply too different for you not to have explicitly and in writing in the plan, the order, or at least an email pointed out by how many centimeters the measurement for the BP has to be smaller.

If there were a phone call X where you explicitly pointed this out, you could ask if he remembers and hope for goodwill. But as the predecessors write: Don't be upset about the amount! Be glad that pipes were not placed incorrectly or even concrete poured incorrectly based on the wrong measurements.

And learn from this for the future to arrange everything in writing, think along yourself because your counterpart will not do it, and double-check everything (which obviously already worked ;)).

Something similar happened to us. We would have had it in writing in an email which plan contains which data, yet the construction workers on site still messed it up. Since I was on site, it was noticed and could be corrected immediately. In the end, I stood next to it for 3 hours and rather checked every measurement (and bought a coffee ;)).

That is building :D

In proportion, a fortunate mistake from which one can wonderfully learn..

To be honest, I would be worried now that the slab builder is doing everything right if he can't even manage to compensate for a few centimeters of insulation or the like over the conduit. I suspected trouble with a defect notice because of something like that...

Honestly, I would be worried now that the slab builder does everything correctly if he can’t even manage to compensate for a few centimeters of insulation or similar over the formwork. I had a bad feeling about a defect notification because of something like that...

I think you didn’t understand the issue. If the surveyor measures incorrect dimensions, that says nothing about the quality of another trade.... And in prefab house construction, the dimensions of the slab are indeed very relevant due to preproduction.

No. The surveyor delivered the fine staking according to surveyor standards. (House dimensions). However, the foundation slab is supposed to be narrower according to the execution plans. And the foundation slab builder fails to form his slab based on the correctly set house dimensions. For me, that is simply a poor performance. And even the shell constructor of my main contractor managed that, whom I generally consider to be a below-average good shell constructor...

Unfortunately, there is no document defining it.

Then the foundation slab builder cannot claim that it is wrong either.

But isn’t it the case with (almost) all prefab houses that the foundation slab is slightly smaller than the house?

Yes, but it varies, sometimes 2cm, sometimes 4cm. And with basements it’s different again. And then there are facing bricks, solid clinker bricks, slabs, plaster, ... And often there are changes between the approval planning and the execution planning.
The foundation slab builder should have formulated precisely what he expected. Did he carry out the earthworks after the rough staking? Then he could have already seen on the rough staking sketch which dimensions the surveyor marked and could have clearly pointed out again which dimensions he expected at the fine staking.

The surveyor redid or corrected the fine staking. 200€

That’s fair. The dispute with the foundation slab builder is really not worth it for that.

Honestly, I would now be worried that the foundation slab builder is doing everything correctly if he can’t even manage to compensate for probably a few cm of insulation or similar over the formwork.

That is indeed worrying. At least he saw that finished dimensions were staked out.

If the surveyor measures wrong dimensions

The surveyor did not measure wrong dimensions. Surveyors and builders just need to agree. Many construction companies want finished dimensions precisely to avoid such misunderstandings. The few centimeters are then measured back on site with a tape measure. Then the strings are not in the way during formwork.
Here the communication simply went badly. Surveyor, client, builder – no one or everyone is to blame.