Rainwater retention basin overflows onto the property

These are the guidelines you should follow! The building authority has already given you its opinion.

No matter which way, whether over legal recourse or over the above-mentioned construction measures, it will first become expensive. I don’t know of any legal protection except ARAG (I believe it only covers up to a dispute value of €10,000) or the insurance that cooperates with Sparkasse, which applies to new construction projects.

Oh dear, that doesn't look good!
Is there going to be a road in between, or does your property border the retention basin directly?

Oh dear, that doesn't look good! Is there going to be a street in between or does your property border directly on the retention basin?

Provided it is approved. To build a concrete wall around your property 1.5 meters high which also extends 2-3 meters deep. Then you should have peace. However, that would not be exactly cheap.

I also wonder how the municipality can say that this is your problem. The retention basin belongs to the municipality. And as a layman, I see it so that they are responsible for ensuring that no water from the basin gets onto your property. Aside from a storm which causes the water level in the basin to rise by a meter or more in a very short time. I am sure if it were your basin and the water was standing on municipal land, the municipality would be at your doorstep faster than you could say water.

I would probably already consult a specialist lawyer now and ask for an assessment.

Otherwise, you probably have no choice but something like what I initially mentioned.