Building permit for building land denied due to flooding

In a village of 2000 inhabitants, there were a few connected gardens. Over 5 years ago, these were still declared as "garden land" and the QM price was accordingly low.

Now this area where the gardens are located has been upgraded from garden land to building land.
This is good for the owners because the value of the property has tripled. :p

Now someone bought 2 gardens there and wanted to build on them.
But the building application was rejected,
with the reason – flood-prone area. :confused:

I have lived there for over 35 years and there was once a flood from a small stream about 30 years ago, which has now almost dried up and since the flood 30 years ago has been straightened. Well, the flood risk is “relative...”
But more important now is why would someone change a property from garden land to building land
And then if someone wants to build, they are not allowed to ...!? :eek:

Moreover, the 2 garden plots were bought/sold at a completely overpriced value!
And this is not just a matter of a few hundred euros, but rather losses in the range of five figures!

What should the new landowner do now?
Can he possibly take legal action against the municipality because they simply changed garden land into building land?