Wild flowing water - What is correct?

So, I have now waded through the legal texts, read judgments, etc.

First, I want to clarify the terrain situation again:

Above us are 2 undeveloped plots on a slope; water from additional undeveloped plots located to the side of these also flows onto them. This water does not fully seep into the side plots with clay soil, then flows onto the undeveloped plots, increases in volume, and then flows to our upper-level neighbor, who unfortunately has paved everything at this point (which he is allowed to do as long as the floor area ratio is observed). He has a soakaway shaft at this spot that is quickly overwhelmed, so the water flows onto the lawn behind our house. The next (lower-level) neighbor has now built a wall (about 40 cm), against which all this water now backs up. In heavy rainfalls, the water would now run into our basement. He now wants to fill up on his side against this, but according to the development plan, this is not allowed. Incidentally, the plan also prohibits connecting basement shafts to the sewer, which not everyone has adhered to. On top of the construction, a hedge is supposed to be planted (which would again impede the water from flowing off once it is fully grown).

In the Water Resources Act (WHG), wild flowing water is defined as follows:

According to § 3 No. 1 WHG, surface water is to be understood as water that flows wild according to natural terrain conditions from springs, meaning it has not yet formed a watercourse. Additionally, § 37 para. 4 WHG stipulates that paras. 1 to 3 also apply to wild flowing water that does not originate from springs. In summary, wild flowing water is thus to be defined as follows:

Wild flowing water is surface water outside a watercourse that either originates from springs or collects on the ground as precipitation or meltwater and flows off following the terrain level.

I would thus count our water under this category. : It is also definitely not wastewater, I have researched that as well.

In summary, § 37 para.1 WHG contains the prohibitions relating to water flow between upper and lower neighbors. An owner is generally not allowed to artificially change the flow of wild flowing water in such a way that lower-lying properties are disturbed and vice versa.

If my lower-level neighbor thus causes the water to back up with a structure, that is not allowed.

Of course, I am not a lawyer either; it’s amazing how you have to struggle through all this to understand it.

According to § 3 No. 1 WHG, surface water is to be understood as water that flows wild from springs following the natural terrain conditions,

Then I will pose a rather heretical question as to whether the construction of the properties has not at least slightly altered the natural terrain conditions and/or the water runoff before construction. If so, then these would no longer be natural terrain conditions and the entire line of reasoning collapses.

Hmm. I come to completely different conclusions with the same sources, sorry. Starting with your upper neighbor, all of those are developed properties and even the other two properties could be paved, this is not clear from the description. But in any case, it is no longer a natural terrain and naturally collecting water. Opposite the upper neighbor, you have the right to demand that he, in whatever way (upgrade the soakaway shaft, reduce the sealing level), prevents the wastewater (= among other things, precipitation water from developed or paved properties) from his property from draining onto your property. I still believe that you must ensure that your wastewater is not drained onto your lower neighbor's property. Ultimately, you have to decide how you behave now. Maybe convene a neighborhood council with all parties involved? Fuchur was faster...

It seems to me you have also read on a relevant legal platform. Perhaps you did not finish reading the article by Mr. Hans-Albert Wegner because it explicitly states:

Important
Natural Water Cycle
What is decisive in any case is that it is surface water that actually flows wild from a source or initially accumulates naturally to then flow off due to the gradient. Therefore, water that does not belong to the natural water cycle, such as water escaping from burst pipes, is just as little covered by § 37 WHG as precipitation water that flows off collected from built-up or paved surfaces. Because the latter is wastewater according to § 54 para. 1 no. 2 WHG, for which the special regulations of §§ 54 to 61 WHG apply.

So first of all, thanks for your objections, which had partly occurred to me while reading the laws. Here are my thoughts on it:

Then I'll ask a rather heretical question whether the construction on the plots has at least slightly changed the natural terrain conditions and/or the water runoff before the construction. If so, then they would no longer be natural terrain conditions and the whole chain of reasoning falls apart.

So the first two plots are undeveloped, therefore not changed. If at all, I have to look at the upper landowner, who has paved over the natural terrain, so it is no longer completely natural. According to my understanding, he would have to dispose of the water that does not infiltrate because of the pavement himself; he does so via the infiltration shaft. But the shaft cannot absorb the water from 2 undeveloped 1400 sqm plots. The source is still to be found on the undeveloped plots. These will remain free for at least another 10 years. We cannot wait that long.

It seems to me that you also read on a relevant legal platform. Perhaps you did not finish reading the article by Mr. Hans-Albert Wegner since it explicitly states:

Yes, that’s exactly where I looked it up. Unfortunately, one cannot read the article to the end without high costs because the point "1.2.3 Changes in water runoff as a result of altered economic land use" would also have interested me. There are rulings where an upper landowner tore down a garage and since then water runs over to the lower landowners. The lower landowners lost their lawsuit back then because it was said that the upper landowner can determine any time how he uses his plot. If he doesn’t have a garage that holds back the water anymore: tough luck.

So again to our case: the water from the upper landowner that falls on the pavement I would classify as precipitation runoff from a built-up area. But the bulk of the water comes from undeveloped plots, so they are unsealed.
------------- Then paragraph 37 applies.

Of course, we all have to agree; nobody wants legal disputes or flooded basements.

In the Journal for Environmental and Planning Law the following is stated:

[...] Nevertheless, this water from precipitation on unpaved surfaces can develop into uncontrolled runoff water, especially when, for example, the upper property can no longer absorb the water from precipitation through natural infiltration and it subsequently flows onto the lower property and causes moisture and damp damage (also to buildings) there.11 This situation can arise, for example, with upper properties when several meters thick clay layers are located beneath this property, so that the water from precipitation takes a long time to infiltrate naturally and the water from precipitation remains standing on the surface of the upper property (because the soil is more or less already saturated with water like a kitchen sponge) and then the water standing on the surface flows onto the lower property due to a simultaneously existing slope. In this case, the uncontrolled runoff water must be diverted if necessary or—if possible—collected in infiltration basins as an intermediate storage between the upper and lower property.[...]

So the respective neighbors could together create infiltration basins, so to speak, on the common boundary. Furthermore, I imagine that each property owner could then create a "bed" around their house on their property, which then in turn flows into the next infiltration basin at the next downhill boundary. If everyone pitches in and helps out with each, this could possibly be done quite quickly. The resulting debris must then be spread by the farmer on his own land. Then he will have also done something.