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

Apparently, however, the problems that have now arisen were not pointed out. Otherwise, the clients would certainly have decided very differently. What use is the question if, as a layperson, I am not told the reason?

Why did the building authority want the soil survey? That always belongs in the hands of the [BU] because of the foundation costs. Didn't you give it to them? Well then: I would also take the path to a lawyer to see if there is a chance of halving the costs. And then install a proper drainage system with a soakaway basin and/or shaft or something similar. Make sure that the neighbors do not drain towards you!!! Someone already wrote this here, but repetition is good. Otherwise: welcome to the forum [emoji847]

If yes, how would you proceed further?

I think the most important hints have already been given (lawyer). Nevertheless, I will still add my two cents, I believe there are still open questions. Am I the only one who does not know the abbreviation OFFEG? Do you mean Oberkante Fertigfußboden? Or something else? From your information (40 m long plot, house at 62.5 m (above sea level?), 1 m slope, 70 cm step) I do not make sense of it. Where does the step come from now? In my opinion, that does not quite fit together or you have not conveyed it clearly. Anyway, not so important, but for yourselves you should describe the problem clearly. What always helps: a sketch, cross-section through the terrain, mark original terrain profile, mark current actual state, mark house. Add some numbers. Done. Works well by hand. Then it helps in my opinion to structure the problem; what is the problem at all? Technically - waterproofing of the house against water (clarified?) - foundation of the house (clear, right?) - step to the neighbor? Legally/financially - what costs actually arise? - contract situation? I hope this helps you, these are just suggestions/questions ... I would go to a specialist lawyer for construction law. Such can be found very easily (Google, Yellow Pages, Bar Association). You can certainly find them much easier than suitable experts. You also need one of those, but maybe the lawyer knows one ... Otherwise, Chamber of Crafts, etc. The lawyer can only clarify the legal part, but you also need a professional/technical clarification of the situation and proposals for solutions where it should go. Someone who can do this must do it for you. You yourselves cannot (which is no shame). With this (then hopefully usable) planning, you can tackle the implementation. Then you will know what it will cost. You certainly have to advance lawyer’s fees and technical consulting/planning and probably you will be left with these costs. In implementation, perhaps a compromise with the general contractor can be reached. But the most important thing is to achieve a permanently viable solution. In my opinion, you have to bite the bullet.

We are now trying to defuse the situation through suitable specialist companies and keep hearing that a lot must have gone wrong in the planning

Are there already concrete proposals on what to do now?

The soil report was commissioned by the construction company and is part of the documents for the building application.

Sorry; I meant to write upper edge of finished floor ground floor. And yes, it means upper edge of finished floor on the ground floor.

Again on the information:
The plot has an incline of 1m from front to back and we built relatively far at the back (about 8m distance to the rear property boundary). Our terrain reference point is given as 62.3m and directly after the splash water area the ground jumps to 63m. We will still have it removed there, but will certainly have to work with retaining walls here. I hope it is now more understandable?

The specific questions for me are
- the sealing (here the construction company assured me that it is sufficient - but I am uncertain and would like this confirmed by someone independent)
- and the additional costs we have to bear in order to shape the terrain and for rainwater drainage

Unfortunately, I cannot find a competent contact person for the specific measures.
I had a civil engineering company here, which said it would only let the water run away from the house by terrain modeling and would not recommend drainage because it has no hold in such silty soil and would constantly be clogged. A landscape gardener was completely overwhelmed by the situation and did not get back to me.
What kind of specialists should I specifically look for?

Today I was able to have a phone call with a specialist lawyer via the Bauherren-Schutzbund. As a first rough assessment, he said the case is not clear and therefore he cannot give an assessment whether it makes sense to sue. He only knows the house building company and knows that no settlements are reached. Thus, there might be a chance in a longer procedure. He would represent me, but honestly he did not sound very confident.

So in a longer proceeding, one might possibly have a chance. He would represent me, but to be honest, he didn't sound very confident.

I believe it would already be an added value (and costs in the range of 300 euros) if a specialist lawyer reads your contracts and explains the content to you. Normally, the very cautious and prudent builder does this before signing, but better late than never :P. How to proceed afterwards ... Whether to have the lawyer draft a letter or not (with or without his letterhead) can still be decided after the consultation.

Which experts should I specifically look for?

Unfortunately, I can't say either. Experts with universal house construction expertise or those who specialize in waterproofing issues. That would be my direction of search ... Civil engineers, landscape gardeners, etc., come into question afterwards as executors; prior to that, in my opinion, someone knowledgeable has to take over the planning. Apparently, your general contractor isn't doing it.

I don't quite understand the problem yet. The planner said that only a small portion of the terrain in the rear area needs to be excavated. But it was clear from the start that something had to be excavated. What is a small portion? Excavating 20 to 70 cm is small to me?

Assuming your house is at 0.00 and the neighbor is at 0.60. I would immediately slope down at the neighbor's side to -0.10 or -0.20 and then slowly let it rise again towards the house to 0.00. Where is the problem?

With other neighbors at 0.20, just go down to -0.20 and then slowly rise to 0.00.

That neighbors are not allowed to drain onto your property is not entirely correct either. If it's due to the natural terrain slope, then it's perfectly legal.

But nothing has to flow towards the house. In general, the ground must be sloped away from the house professionally in all directions by at least 1-2%. Not just the terrace. This is stated, for example, in the guidelines and technical information of the plaster/insulation manufacturers.