Raised embankment by neighbors, allowed? What to do?

Hello, maybe someone here can help us with a specific problem. We have a property on a slope. Our house is built into the slope. The entrance is at street level, the basement is practically half in the slope. In front of the basement area, the property was leveled about 4 meters wide up to the property boundary. From there, the slope continued downward. The neighbor who owns the property below has a narrow driveway to the right of our property and drives down the slope to his property. About 4-5 years ago, he slightly filled in the slope on his side to have a flat area at the same level as our basement / lower property area. It should also be mentioned that he has only used the property so far for storage and recreational purposes, as he owns another property below that is developed. Now he has had L-shaped stones delivered. He plans to place them along our boundary and thus raise his part of this essentially flat area by about 50 cm and pave it for parking spaces. We are now concerned that water might flow from there into the lower part of our property and not be able to drain, since there is the slope there. This area is already very damp due to the clay soil. Our question is, is it allowed to simply raise a natural slope? Since this sounds a bit complicated, please find attached a sketch for understanding. It concerns the area marked in red.

Basically, rainwater from your property should also remain on it. In this respect, I see little leeway at first, and what is now being implemented structurally is what has legally been in effect for a longer time. Of course, the neighbor must design his paved area so that it does not drain into your property. That means the 2% slope towards his property.

Our neighbor is a do-it-yourself hobbyist who probably doesn't care much about drainage regulations. Actually, this red area is his building window and he would have to keep a 3 m distance from the boundary with the construction, can he even pave without permission and create parking spaces up to the boundary? Thus, vintage trucks might possibly be parked directly at the boundary.

The regulations on drainage also apply to hobbyists. If necessary, you can take legal action, but it will certainly be a lengthy process. Therefore, I would try to talk to the neighbor. Parking spaces and garages are usually privileged at the boundary, but development plans can regulate this differently.

Okay, thank you very much first of all. I actually thought that you are not allowed to raise plots just like that, but that also means conversely that if he fills up 50 cm and later still builds there, his house might have to be 50 cm lower from ground level? On our side, 4.50 m is allowed on the uphill side.

What you are allowed to do and how you must build depends on your federal state on the one hand and on the development plan on the other. For example, we have specified the ground floor floor height but are allowed to deviate from it by up to 80cm. Building height is regulated separately. It also depends on the details (what is the exact reference point and how it is calculated); without knowing the development plan, I would not make any statement.