Garden drainage during heavy rain like recently in NRW

Underground, a kind of infiltration trench. Of course, the pump must have power. But that is the smallest problem. Wastewater pumps are available in all sizes. Otherwise, there are also mobile water barriers...

Here is some feedback. We will now have a hole dug with an excavator about 1.50 meters wide, equally deep, and approximately 8 meters long up to the terrace. Gravel will be put in there, then topsoil, and then grass again. A drainage pipe will be laid in the splash guard, which will be connected to the roof drainage. Additionally, we will build a “shaft” in which we can place a pump just in case the water rises again. However, the landscaping contractor was very clear that these are only measures that delay things depending on the amount of rain. :( There is probably not much more that can be done since all the water flows to us from the KiGa. Now we have to hope that they are willing to take some action as well. At least the church is willing to talk. That should be a good start.

If you have to do excavation there anyway, would there be a possibility to combine it with a cistern? That way you could at least collect the water partly for irrigation and might get back some of the investment over the years. But I'm not an expert, please don't stone me if this is a stupid idea.

because all the water flows to us from the KiGa.

Can't you build a

small 20-30cm flood wall

at the border to the Kiga?

The kindergarten is not allowed to simply drain onto your property. That must be addressed first.

I am too lazy to quote you all.
[Cistern] was also my idea, unfortunately we have a problem there with the distance to the boundary.
We also considered a flood wall. But it would have to be built on the side of the [KiGas] so that it helps. Nevertheless, we have ideas there. Our garden and landscaping contractor has done something like that before with root barriers for bamboo for a client. That stuff can apparently hold back quite a lot.
Regarding 3. [KiGa], see above. We also want to address that. However, there seems to be little legal recourse there. We also don't necessarily want to risk it, as our little one is accommodated there.
As mentioned, they are willing to tackle the issue together. I think that is worth a lot.