Is there a law regarding drainage?

The danger when plugging up is that the water will find another way. (Don’t know how tight or permeable such clay pipes are). If it should back up 1m and then seep out from the clay pipe again. Not quite sure. But tendentially, I would plug it up anyway.

Hello dear members, When sealing our basement exterior walls and opening the excavation pit, we noticed that a drainage pipe (clay) from the neighboring property leads directly onto our property, or directly onto our basement wall.

1. I hope your excavation pit was constructed professionally; otherwise, your house might collapse. Excavations near foundations/house walls should not be carried out by laypersons. 2. Drainage is not permitted everywhere; is it allowed in your area? It is usually stated in the local sewage regulations. 3. Everyone is generally responsible for ensuring that surface water stays on their own property, meaning drainage from your neighbor to you is not permitted unless they have a registered right to do so. 4. Request the neighbors to promptly remove it.

1. A contractor has opened. It is "only" about 3m in length on one side of the house. The rest is still closed, or was once opened and closed again. 2. At our place, the drainage system can be connected to the public sewer. 3. According to the building authority, there are no encumbrances. I could also inquire again at the land registry office. ([Beim Notartermin wurden damals keine weitere Dienstbarkeiten oder Baulasten erwähnt.]) 4. Nothing else remains (especially because no willingness to compromise or dialogue was expressed in the first conversation.

when sealing our cellar exterior walls

I even go as far as to say: the cause of your damp cellar should now have been found. If the neighbor no longer drains his property through your former construction pit, your wet cellar will certainly be resolved, probably even without you having to spend a fortune on elaborate sealing.

Then coming up with an invented "drainage law" is hardly surpassable in audacity. Transboundary natural watercourses and overflows must not simply be blocked and dammed at the border, that is true. But that does not cover "I lay a drainage pipe in the neighbor's construction pit."

Joking aside:

Even if the neighbor initially reacted stupidly - it is foolish to escalate the situation further. After all, you want to live in the house for a few years without having an ongoing dispute with a neighbor.
Goal congruence can be created: Everyone wants to get rid of their water without suffering damage. Everyone potentially prefers an amicable solution over a dispute. No one wants to lose money.
Solution approach: The water is allowed to go into the sewage system. That is where it should be directed.
Compulsion to act: Your house is damp, the neighbor’s water unlawfully reaches your property.
Consequences of inaction: Expensive repairs for you, costly damages for your neighbor.
Possible concession: You permit a water path over your property for the sewer connection.

If that is not possible: you can always still argue.