Single-family house construction project on a slope -> possibilities to secure the slope

Great, thank you very much for the answers and support.

I voted yesterday that we will commission a retaining wall in the north already during construction and initially leave the retaining wall in the south as it is and then commission it afterwards.

In the pictures, I clearly see regarding §34: flat roof no

Even though - for whatever reason, unfortunately passed me by ((just like with (whom I had unfortunately asked elsewhere without receiving a response)) and it is very very unfortunate - is no longer here to provide advice and support, it should especially resonate with you, who is said to have a Sheldon Cooper-like eidetic memory, to have his recurring words on this topic of §34 still ringing in your ears: the roof shape has no relevance for integration into the surrounding buildings! ;)

I voted yesterday that we will commission a retaining wall in the north during construction and initially leave the retaining wall in the south as is and then commission it later.

If it is absolutely out of the question for you to level the terrain differently than shown in the drawing in the initial post, then the proposed retaining walls will be non-negotiable both in terms of their nature and the timing of their installation, but urgently advisable. However, given the already not insignificant costs estimated for this, in my opinion, you should be more open to alternatives. Raising a house because one has classified a slope protection as retrofittable will be even more expensive.

the roof shape has no relevance for fitting into the surrounding buildings!

But the pitch, and indirectly the shape: with gable roofs of, for example, 35 degrees in the neighborhood, a 30 or 40 degree hip roof, clipped gable roof, or even shed roof is feasible, but a flat roof aka 2 degree shed roof is not.

But the pitch, and by this detour the shape after all: with gable roofs of, for example, 35 degrees in the neighborhood, a 30 or 40-degree hip roof, half-hip roof, or even a shed roof is feasible, but a flat roof alias 2-degree shed roof is not.

I have to contradict again, so to speak on behalf of ;) :

This is a §34 area according to the Building Code, there is no obligation to conform regarding roof pitches or shapes.

And also as a fundamental addition:

What the obligation to conform relates to is precisely stated in the law:
- Type of structural use (residential area ... industrial area)
- Degree of structural use (utilization of the plot based on site coverage ratio and floor space index, eaves and ridge heights, number of stories)
- Building type (open, closed, deviating)
- Plot area to be built upon (actual building limits / lines)

I have to, so to speak, contradict once again on behalf of @Escroda ;)

Nice, then the leaning tower of Pisa can at least have a flat roof – when deriving the floor area ratio from "one-and-a-half-storey buildings" however possibly with a recessed upper storey, here at least without strictly defined setbacks.

From when do I actually need retaining walls? Or rather, when do I have to support the slope?

You need to support the slope so that it does not slide into the valley and so that your house is not damaged if something slips. A house exerts pressure on the slope in the immediate vicinity. The slope must withstand this and may need support if necessary.

If the slope is far enough from the house, a slope of up to 60 degrees is sufficient. As an alternative to L-shaped stones, natural stone can also be used, for example. It is also possible not to catch the entire width but to leave gaps in between, thereby shaping the terrain a bit and preventing the construction from appearing too massive.