Must developers indicate immense embankment height/costs?

Yes, we have taken that into account, neighbors both similarly high, garage to garage at the same level, etc., that all fits. Slope towards the street.

But what use is your answer now to the OP?

Right, I overlooked your solution, where does it say that again?

What we didn’t know, however, is the specified building height of the house, which is above the original ground level of the plot, and because of that, our garden area had to be filled up again

Did this fun cost 30,000 euros? Hmm....... I know of quite different amounts, so the questioner got off "cheap" this time.
I am surprised that for 50 years builders keep making the same mistake: They don’t have an architect and thus waive his consulting and advisory duties. Instead, they buy from the developer, who does not have such duties and relies on the usual parties involved in construction, which are required for every project, being replaced one-to-one. Then the builder does not even have an advisor who checks the construction contract along with all attachments, no plausibility checks take place and the project is not professionally launched – only to the extent that it satisfies the interests and agreed construction services of the developer. The builder does not even have a safety and health coordinator and will be surprised if he is held criminally liable as a consequence of an accident and the BG (professional accident insurance) claims compensation from him regarding the accident costs (for example, a femoral neck fracture costs 80,000 euros). Whether the property is free of unexploded ordnance does not interest the builder either (it might trigger a fee of 70 euros), and that is why bombs from aerial bombardments are often found under newly built houses. The list of what the builder waives could go on. The catch, however, is that all waivers are risky and when the risk occurs, the matter becomes terribly expensive.