Embankment of land in a "flood" area

Why? In my opinion, only the area for the house, (terrace?) and the driveway are to be raised.

As far as I can read from the development plan, the entire property must be raised to the level of the street:

"Adjustment to the traffic areas: In the immediate vicinity of the public traffic and green areas, the finished level of the property edges must be adapted to the level of the traffic and green areas."

Besides the financial aspect, I also see the emotional side. What is it like to wait for a flood, watching the water level with anxious eyes? What is it like to have lost a lot, even if it is somehow compensated?

The classification into HQ areas is statistical and is based exclusively on values from the past. Among the 25 largest rivers in Germany, there is not a single one that has not had at least one HQ50 flood event in recent years; several even had two or three. For me, this reveals a mathematical peculiarity. Bavaria has issued recommendations to exceed the standards associated with the HQ classifications by at least 15% in order to take changes due to climate change into account.

I am not an expert. In a floodplain area classified as extreme, I would only build floating structures. If the area is additionally at risk of overflow, not even that. For the risk taken, I would expect a special increase in quality of life, such as an unobstructed view of the water. In the case of the location of the property in question, this is not the case. My assessment is not exclusively rational, no "right" or "wrong."

I consider it economically questionable that land is designated for building in areas classified as HQ100extreme in the first place.

as far as I can read from the development plan, the entire property must be raised to the level of the street: “Adjustment to the traffic areas: In the immediate vicinity of the public traffic and green areas, the finished level of the property edges must be adjusted to the level of the traffic and green areas.”

That also makes sense for many reasons. And in the end it will even be significantly more expensive if you only raise the areas for the house, garage, terrace, and paths and retain the rest with 1.5-1.8m high L-stones, a dry stone wall, or another method. Sloping down a 1.5m drop alone costs an enormous amount of space anyway because you should make a maximum slope of 30° and also carries the risk that rainwater will run from the street onto the property if the drainage system fails during heavy rain.

Let's see if the municipality has a heavy rain management plan. Different values than a hundred-year flood are used there. The Danube villages are flood-tested, but what if it comes far above or faster than a normal flood. Just as an example, our little stream with about 15-20 cm water height during flood about 75 cm can become 5 m high in heavy rain according to the model, and that in a short time. Without insurance options, I would not build. Therefore, clarify this first.

Technical measures, insurance, all fine. But the most important factor was named by Wiltshire. Which type are you or are you all?

I have a colleague who sat next to me when the flood was already flowing down his street, sandbag barriers were being built, and he said to me when I asked if he didn’t want to drive home, "I’ll manage here somehow on my own... What am I supposed to do? Scoop away the floodwater with a bucket? Stand in front of the dam with tears in my eyes? In the worst case, you just have to renovate..." If you are that type, build there.

If you are the type who already gets a stomach ache when the warning card shows level 2 or 3 for heavy rain, then don’t do it. Also think of your better half. We had water standing up to the bottom edge of the slab on the property in the year when Lower Saxony and Saxony-Anhalt sank in floodwater. I was relaxed, the slab is waterproof, no danger... for my better half a world collapsed that day... if there had been a real flood danger for us, we would probably be living somewhere else by now...

Technical measures are expensive, insurance only steps in after the disaster has already happened. The personal component is decisive. It’s no use to you if the insurance pays for the renovation but your family no longer wants to live there and leaves you sitting there alone.

Oh oh.

So. Before I would even think about buying the property, I would clarify a whole series of things.

1. Insurance possibilities:
Online portals are not very informative. In risk situations, individual assessments are often made, and depending on the insurance, flood damage is often excluded. (I do remember that a legal change is supposed to come, making it mandatory to be able to take out an elemental insurance - but I don’t know how the implementation is proceeding.)
So: Go to the insurance company where you have so far been the best customer and have it clarified there.

2. Soil survey on the property exactly where you want to build. Sorry, but relying on the city soil survey based on preliminary signs (whose boreholes might be somewhere else) is not a good idea.

3. Really take a deep look inside yourself, as others already wrote, and ask yourself how well or poorly you can live with the risk.
(In this context: How high would the water stand on the property in an HQ1000 flood? Would it be more or less standing water, or flowing water?)

4. If you can still buy the property then, try to find out which company has already built a lot in the area and ask there about experiences and cost estimates. Especially in rural areas, there are usually a few “usual suspects” who have experience with the local conditions and can give at least somewhat appropriate assessments.

I’m afraid: It will be expensive.