Half of the roof does not drain - rework not possible - options?

No idea. From my experience with GU(Ü)s, they are very stingy with money. Better to request some other service. You chase the money for ages. Theoretically, you are allowed to withhold twice the costs necessary to fix the defects. And only if it was his fault. I am also skeptical about that here. He will take the position that you said you wanted the cistern placed there, and then he just did it that way. If the drainage planning was not also part of the contract, he also has no obligation to check if it makes sense. A good one notices it beforehand and then asks, but he is not obligated to do so.

Is no rainwater coming into the cistern anymore? Then it was installed incorrectly. The excavator operator/contractor should have reacted before they installed it incorrectly. Then I ask whether it even has to be paid for.

Relocating the cistern is not an option?

If the drainageplanning was not also part of the contract, he is not obliged to check whether it makes sense.

It is part of the contract or the work plan.

Does no rainwater reach the cistern at all now? Then it was installed incorrectly. The excavator operator/contractor should have reacted before they installed it incorrectly. Then I question whether it even has to be paid for.

Moving the cistern is not an option?

Yes, from the other half of the roof.

No. I see moving it as even less of an option technically.

The cistern was part of a contract for work. I still have a retention.

OK, then he definitely messed up. I would still assume that the BU will not want to pay an appropriate amount and therefore first ask how he intends to fix the defect. So far, he has only explained how the defect came about.

Just for your information. Today the GU was here with a small excavator. The pipe could be rerouted and integrated into the cistern. It took a little while, but now I’m glad that it fits and the matter is settled.