Neighbor's drainage on our property

Dear forum,

since last week we have finally started digging. First, our garage is to be built directly next to the neighbor's garage into the slope. Our plot starts right at his garage (border construction utilized down to the last mm). The mentioned neighbor has been working on his house for years and apparently has hardly any money left (all the other houses have been finished for 5-10 years). Our site manager already noticed during the first inspection that his garage wall is too thin and probably has no frost protection strip. That means +10k for underpinning for us... During the earthworks, we have now discovered a drainage (slope water) from the neighbor on our property. Exactly where it lies, our garage is supposed to be (slightly lower than his garage). I have attached a few pictures. Am I correct in assuming that the neighbor is responsible for removing this thing or bearing any additional costs? Do you have any ideas on the best way to deal with this? Of course, we do not want the slope water to flow freely either. According to the neighbor, there is an identical drainage on the other side of his property, so that there is a drainage provided.

If I am not mistaken, it is basically prohibited everywhere to direct your own rainwater onto a neighbor's property. At least I know that it is forbidden here - and I would find it illogical if it were allowed anywhere in Germany. Go to him and remind him of his pipe. He has certainly only placed it there temporarily and for a limited time and now forgotten about it.

Now, to me that does not necessarily look like a provisorium. But even then, without the permission of the property owner, something like that should not have been laid there.

The aforementioned neighbor has been building on his house for years and apparently has hardly any money left (all the other houses have been finished for 5-10 years).

This does not necessarily have to be true. I know construction projects that run for years because the builders finance the construction from their ongoing income. That then takes its time.

Our construction manager already determined at the first inspection that his garage wall is too thin and probably does not have a frost skirt either. That means +10k for underpinning for us...

Would you like to explain to me what your CM means by "too thin"? From the pictures, I cannot tell whether a frost skirt has been installed under the neighbor’s garage.

During the earthworks, we have now discovered a drainage system (hillside water) of the neighbor on our property... Am I correct in assuming that the neighbor is responsible for removing that thing or bearing any additional costs?

Yes, provided that no building encumbrance/easement for his drainage is registered on your property. However, I find it strange that the pipe is hanging around so openly. Or was it detached from the rest of the pipeline during your earthworks?

Do you have any ideas on how we should best deal with this?

Talk! Arrange an appointment with the neighbor, at which your CM should also participate, and then see how you can settle the matter - preferably amicably. Don’t forget the topic of the garage + frost skirt. Rhine regards