Plot in flood protection area - Not recommended?

How do you come up with the idea of wanting to build in a flood plain when your first question is about the costs???

...especially 0.4 / 0.8 - mind you: depending on whether the slope is fully included there.

OT: Since when does a slope count towards the floor area ratio? I've never heard that before. Is that normal?

Well, your hillside location isn't much "better" than a property threatened by a hypothetical flood - even though all actuarial mathematics can occasionally be evaluated. I myself would only have the Rhine 50m in front of my door if the dike broke. My residential area, developed about one hundred and twenty years ago - that is before the dike was built - is therefore at a higher elevation. We all have basements, at least I know of no exception.

For several years now, prospective builders have been so hyped up that one's ignition coil can soon glow if one has been searching for quite some time. Anyone who has seen many sick pigeons gladly takes the sparrow in the hand. And 550 sqm is not little nowadays, especially 0.4 / 0.8 - mind you: depending on whether the slope is fully included.

Sorry, but this is pure cynicism. How does he come up with that idea? Probably there are simply no alternatives at the moment. Do you think there is always, everywhere the perfect plot of land for everyone? And also at any price?

That completely ignores reality. People have been building by rivers for thousands of years, they build on Vesuvius, above coal mines and elsewhere. If no one were allowed to build anywhere where there is a chance that every 100-200 years the house might be wiped out or fall victim to nature in some other way, then half of the existing buildings wouldn’t even exist. The probability does indeed make a big difference. In Passau on the Danube, the basement floods every 2 years, in the building area of the OP (original poster) maybe not until the next millennium. And therefore he shouldn't build there today? The HQExtrem area also means that you are on the edge of a possible flood. That means you just have some water in the basement, not 3 meters of mudflow up to the first floor. That’s certainly not great, but it’s also not as if the whole house collapses because of it.