Our house, a massive city villa, was planned in depth

Okay, thanks for the assessment. Then I will do that!

The house construction company of course denies all responsibility – we would have confirmed the highest point.

Well, then where exactly is the planning error supposed to lie (and by whom, actually)? It was discussed and agreed upon jointly. Did you expect any clarification?

Was there a soil survey, and was this known to the construction company? What was contractually agreed regarding this? The property was presumably also owned by you (not just possession). Then it presumably was not a "property developer" (who sells land with house, but only a "construction company" for house building (and not also the sale of the land)).

The construction company regularly owes only the house construction, possibly often as a standard still included in the price a clearing of up to 30 cm of top layer. The drainage of the house and building ground is – subject to the unknown exact contractual situation – presumably your responsibility. I do not see a duty to inform (for any alleged failure to clarify). How should the construction company know the building ground without X-ray vision?

As a precaution, see a (specialist) lawyer who will check the contractual situation and should be able to give a thorough assessment. Good luck!

The soil survey existed. It was commissioned by the construction company and signed before the building permit application.

Thanks also for your explanation. I had feared that the legal situation would look bad for us.

This was poorly planned, was there no surveyor? Was there an on-site appointment?

Get professional help because it will actually get more expensive.

But now it’s too late!

Nevertheless, I want to ease your fear.

What do you think people who live on a slope do? They also have neighbors who are higher than themselves!
So build a wall
And a neighbor should not drain onto your property.

Draining onto your property is also possible through a soakaway pit. It just costs money but it works.

Thank you for your words. Yes, we are currently trying to find companies that are familiar with [Grundstücksentwässerung]. So far, no one has really dared to take it on and the recommendations are very different; some say drainage and some say definitely not drainage,...

There was a surveyor. But he just executed stubbornly. Everything executed is correct; only the planning is "not correct."

There was no on-site appointment - the planner did not mention the soil survey at all in the planning process. I am not even sure if she had it. We had to submit it to the building authority afterwards because it was not in the documents.

I am simply shocked and annoyed by the quality of the planning! I have heard from neighbors and friends and thus from various construction companies that there was very thorough advice regarding the property conditions and rainwater drainage etc.

I have one more question, if the house were about 70cm higher now, would you have liked access via stairs? But to put the blame solely on the planner is not right either. You were asked and you agreed.