Craftsman drills into electrical line - who is liable?

You seriously want legal advice from the forum? You do know that this is reserved for professions that provide advice.

If I remember the law topic in my further training correctly, the craftsman is liable for damages he causes.
But whether that is overridden by your sentence "ok, let’s do it that way"? No idea.
Maybe he also said "At your own risk," and you can’t/won’t remember that.
So if you want to know who is liable, a lawyer or a court will have to decide that.
However, I suspect that the electrician with shared costs is the cheaper option.
Especially since it then does not have to be clarified whether it was actually a craftsman service or just pure "neighborly help" offered via MyHammer or similar.

I'll throw the electrician installing the new build into the ring. If they violated the installation guidelines while laying the cable and this violation was not visible during acceptance, they might still be held accountable. Warning: absolute layperson opinion!

I believe an offer via Myhammer is a pretty solid indication that it is a professional execution. Classified ads, if the original poster placed an ad and the craftsman just responded, is more difficult, true.

I believe an offer via Myhammer is a pretty clear indication that it is a commercial execution. Classified ads, when the original poster has placed the ad and the craftsman only responded, is more difficult, true.

It could very well be a craftsman, I just don't know what was agreed afterwards regarding the execution. The OP has not commented on the remark that Traumfaenger already addressed in #2. With my contribution I only wanted to say that executions "as a favor," which are not actually favors, can have negative consequences for both the contractor and the client if it then ends up being decided in court.

I'll throw the electrician installing the new build into the ring. If he violated the installation guidelines while laying the cable and this violation was not visible during acceptance, he could possibly still be held accountable. Warning: absolute layman's opinion!

I believe that an offer via Myhammer is a pretty sure indication that it is a commercial execution.
Classifieds, if the original poster placed the ad and the craftsman just responded, is more difficult, that's true.

I see the issue with the installation directions (installation zones) similarly. However, it is a difficult topic. It depends on what is stated in the electrician’s contract. If it says installation according to VDE or DIN18015, you can rely on that. Provided the cables are indeed incorrectly installed. If it only states "execution according to recognized rules of technology etc." it is difficult because neither DIN nor VDE are mandatory to comply with. They are guidelines and not laws - unfortunately.

Otherwise, I would say, tough luck. After all, you approved the position.

A drilled-through cable is annoying but still not the end of the world.

Yes, my recommendation would also be to aim for a pragmatic solution. Offer to share the costs with him.

Before drilling holes in the wall at places where cables are suspected, one could gain certainty with a cable detector. Costs 20 euros.

But if 17 cm of Styrofoam has been glued to the relevant exterior wall first, the thing is just a better guessing device.