Wild flowing water - What is correct?

(...) Yes, and I agree with you 100%. Everyone knew that the meadows that were there before had always been waterlogged, the municipality should have done something about it. (...)

So you could have known. Fine, the issue is closed. I fell for it too, chin up, straighten your crown, keep going.

Regarding complaints to the municipality and the development planner, I can't give you much hope either. For almost two years now, (extremely persistent) people have been complaining because stormwater runs onto their properties from the private roads during heavy rain like a flood... they are now allowed to pump the puddles into the sewage system, nothing more came of it... this has been going on for a long time and is incredibly nerve-wracking...

I just don't quite understand. Why aren’t you allowed to build a wall to the neighbor who is above you? They should be grateful, because actually they are supposed to costly secure their terrain so that the slope doesn’t eventually move onto your land? Here neighbors are currently having the completely opposite debate, sometimes fiercely, because the higher-lying neighbor absolutely does not want to secure their terrain... "I’ll cut down your slope, 50% on mine. 50% on your property, okay?" … no. I don’t want your stuff. Period...

I am in a situation somewhat similar to yours... My property lies 80% below street level. I was aware of that, I built that way. Everyone is happy. Translated to your situation this would mean that I am allowed to forward all the water that I get from the street onto my property to the neighbor. They will be grateful...

Why? It was clear to me that I lie lower than the street and that the mess runs onto my land? You just have to stand on your property and look around cleverly 360°? The assumption "The people around me will fulfill their duties"... of course that is pretty naive. Some go bankrupt during construction. Some construction company goes bankrupt. Sometimes someone dies, people separate or just have neither the desire nor the time nor the money...

You have, I think, 2 options… you approach the municipality, complain about the development plan, badmouth your neighbors and fight a little promising battle for years. Or you build a wall and have solved the problem for the time being, while your neighbor fights the little promising battle.

The best way, as mentioned, would be to work out a plan together. And not necessarily to take action against the authority, because that will not bring a quick solution. Let him build a wall, but in such a way that the water is directed around the wall and on his property around his house; do this so that everyone additionally delays the time with basins and pits until something really overflows. If everyone pitches in, it will work out. Alternatively, or additionally: Is the owner of the undeveloped properties known? If yes, try approaching him to see if you can dig together on his property to hold back the water or even better let it seep away. How thick is the clay layer approximately, can that be derived from your soil report? If it is only up to 2m, you could break through this layer on the undeveloped properties (groundwater is probably not right at the top of the slope) and thus increase the infiltration capacity there. Additionally, dig out a huge basin and pile the excavation as a horseshoe-shaped embankment against the slope (not too high, so that there isn’t a mudslide).

So you could have known. Okay, the issue is settled. I also fell for it, chin up, fix your crown, let’s move on.

You misunderstand. Our property is located uphill, so I never had a problem before the walls, therefore I couldn’t have known.

Why am I not allowed to build a wall? See a few posts earlier, it is not permitted. §37 WHG.
A distinction must be made whether the water comes from precipitation on one’s own property (which one must drain oneself) or whether the water comes from somewhere else. Here it comes from a field.
I am liable if the neighbor above me ends up with a flooded basement because I built a wall towards him.

Yes, and those further down, like you, built below street level but took no measures at all. What measures have you taken to divert the water from the street? I’d be interested to know....

The best would be, as I said, to work out a plan together. And not necessarily to take action against the authorities, because that won’t lead to a quick solution.
Let him build a wall, but in such a way that the water is diverted around the wall and directed onto his property around his house, with all parties additionally using swales and pits to delay the water until it really overflows. If everyone pitches in, it will work.
Alternatively, or additionally: Is the owner of the undeveloped properties known? If yes, approach them to see if you can dig together on their property to hold back the water or, even better, make it infiltrate. How thick is the clay layer approximately, can this be deduced from your soil survey? If it’s only about 2 m, one could break through that layer on the undeveloped property (since groundwater is probably not right at the top of the slope) and thus increase infiltration performance there. Additionally, dig a huge swale and heap the excavation as a horseshoe-shaped embankment against the slope (not too high, so that there’s no mudslide).

Currently, the wall is continuous where the water would have to pass and also sealed off so that absolutely nothing gets through. Of course, this cannot remain as it is, and that’s why we will propose removing the wall at the end of the property so the water can drain there. But before proposing something like that, one must of course know the legal situation. I can’t just say: remove the wall there if he is allowed to build it (but he isn’t), or build another one uphill if it’s not allowed.

The neighbor above us already made your second suggestion; everyone would have to cooperate for that. Someone here definitely has an excavator, etc. I will bring this up in the discussion with both neighbors on the weekend.

House drains must not be connected to the sewage system.

A discharge of surface water from external catchment areas separate from the sewage system should be sought. Any existing drains must be restored to be suitable for discharge if necessary.

Sealing the ground surface should be avoided. Parking spaces and property driveways should be designed to be as permeable as possible."

That sounds like recommendations but not obligations. For me, the question would be whether the municipality can be obliged to enforce or create a solution at that point. After all, the problem was already identified during the planning of the development area without precautions being taken. Now you have the mess, so to speak. For me, this would be part of the development of the properties and thus to be paid for by the municipality if they sold the properties to you as fully developed.

That sounds like recommendations but not obligations. The question for me would be whether the municipality can be required at this point to enforce or create a solution themselves. After all, the problem was already identified during the planning of the development area, without any precautions being taken. Now, so to speak, you have the mess. For me, this would be part of the development of the plots and thus to be paid for by the municipality if they sold the plots to you as fully developed.

It would probably end up that if something were done, everyone in the development area would have to pay in addition to the original development costs. The municipality probably won't just pay for it because they planned poorly 5 years ago.

We have now had a conversation with both neighbors, and the lower property owner has agreed to tear down half of the wall. We will then create a soakaway on our side. We cannot do more at this point; a large solution is needed for the entire development area.