How high does the house rise - how high to pile up earth (cellar depth low)?

There are two problems when it comes to water. One is erosion during heavy rain. The other problem is that the soil becomes saturated and mushy after a few days of continuous rain, so it no longer holds well on the slope. How quickly this becomes a problem naturally also depends heavily on the soil itself.

The problems make sense. How can one prevent this here? Is there a solution that the garden landscaper must implement to have nothing to worry about?

A slope perhaps 1 meter high is not uncommon, meaning this problem occurs many times over, right?

The soil we would get for backfilling is clayey. Would clay be advantageous here or rather disadvantageous?

When I saw this, the first thing I thought was that this thread should be pinned at the top as a deterrent example for all roof terrace owners.

Maybe I overlooked or suppressed it: What does the contract say about the outdoor facilities? Does the general contractor handle them completely? Does he implement the solution as shown in the plans if you say nothing? What exactly have you agreed upon?

When I saw this, my first thought was that this thread should be pinned at the top as a deterrent example for all slope owners. Maybe I overlooked or repressed it: What does the contract say about the outdoor facilities? Does the general contractor handle them completely? Will he implement the solution as shown in the plans if you say nothing? What exactly have you agreed upon?

No, outdoor facilities are not part of the contract – except for the one mentioned retaining wall for the edge of the terrace on the verge side.

I would rebuild all of this with Lego. Then you immediately see where it gets steep, where a wall needs to go, or where it simply doesn’t work, the first thought ;)

This is meant quite seriously! 3D isn’t for everyone ;)

Here is a constructive suggestion: Pile up 1 meter of soil and then secure it with L stones. That should be quite feasible. Plaster the house at the spots that will then be exposed and create a staircase from the living room and entrance area to the garden. A staircase with one meter should not be too disruptive. And I hope you were allowed to build two full floors, because if the development plan stipulates a single-story construction, you might have to fear a prohibition of use due to the basement height. Because then you would have two floors with such a high basement.

No, exterior facilities are not part of the contract - except for the one mentioned retaining wall closing off the terrace on the verge side.

That was to be feared. So a total disaster. Honestly, I see a clear case of planning and consulting failure here. It is not even certain whether this still counts as a basement or already as a full floor and how this complies with the development plan. The question that arises for me is whether the general contractor should just be released from responsibility here. Correcting this mess will in my opinion cost around 70 to 100K - depending on the execution. Simply "piling clay" won't be enough. First, drainage must be planned. That manhole cover in the front yard can't just be buried either, can it? Clay can hardly absorb water. That means rainwater would run along this "mass" and collect at the lowest point. Depending on the slope, this can quickly lead to landslides, underwashings, or small streams suddenly flooding the neighbor's house, etc. Nothing that is supposed to stand firmly can be built on freshly piled natural soil. Also no terraces - especially not at height. So I would frankly say, this looks bad. To save this chaos, I would probably pile the 40 or 50 cm that are unproblematic - up to the bottom edges of the windows. From there I would work with stairs. Around the patio doors I would build a narrow continuous balcony around the corner, as is customary in the Alps. From there a staircase down to the actual terrace and into the garden. With some luck, that's only 5 to 6 steps.