Base plate incorrectly positioned

If there is a contract for work, the success is "promised." If the success consists in constructing a base slab of quality X at position Y, then X and Y should be correct. It is clear, of course, that the structural builder sees it differently...

See it like Octrineddy, especially if the 20cm space was planned for other things (e.g. carport). That the carport then becomes "only" 20cm narrower doesn't matter. "But the effort is not proportional to the 'suffering'" cannot really be said like that. And from when is it "real suffering"? Just threatening the builder with a lawsuit (whether you want that or not and whether that's the proper way or not is another question) will make him correct his mistake. So he "has to" in a way after all – maybe not voluntarily or according to the law, but it is more than unlikely that the builder will risk it. OP was able to resolve it, which would also have been the only solution for me.

In my experience, almost no one does that voluntarily ... was very surprised to see that the neighbors managed to have the craftsman repave the driveway because it was uneven ... We always only heard: - that's how we always do it - that's how it is - It's okay according to DIN In the end, we really didn't feel like it anymore and reworked many things ourselves

...with us it was a measly 50cm

Datjibbetjajanit. That would be a violation of Art. 3 of Et Rheinisch Jrundjesetz Et hätt noch emmer joot jejange

He has to. He received plans beforehand but did not work according to them afterwards

Yep. He even decided that on his own. He said he prefers that to getting annoyed with the general contractor and the client

Threatening the shell builder alone with a lawsuit (whether you want it or not and whether it’s the proper way or not is another matter) will motivate him to correct his mistake.

I would refrain from threats!

So he "must" in a way after all – maybe not voluntarily or according to the law,

Having the right and getting the right are two different things. That the shell builder corrects the error on his own is great and a matter of course for the owner. But the fact is that if he had let it come to a legal examination, the instance would weigh everything up! Damage, hardship, effort, etc. And then the original poster would very likely have to build on the existing slab but would have received financial compensation. The decision would probably be different if it were now 2.80 meters. Then not only the hardship of the individual original poster would be at issue, but also a violation of general regulations. But as said: would have, if, and but. It’s great that the shell builder reacted positively.