Demolition and redevelopment on a steep slope - assessment?

Leave the old wall on the slope side standing, it saves you from securing it during the construction phase. Many advised us against it as well, so that nothing slips.

There are actually about 1.5 old walls per property or 3 walls in total, which presumably do not form a nice straight line into the slope. I don’t know exactly how slope stabilization during the construction phase looks in practice, but the new walls will tend to have to go roughly where the old ones currently stand, since the building area will be used quite maximally in depth.

The earthworks and demolition costs could be about right. Still, I would recommend allowing a buffer. At the latest, the outdoor area will take a hit.

We are definitely planning a buffer. My concern is rather that I might need not just a 50% buffer but more like 200% or similar for the mentioned works.

The structural engineering can still cause a significant increase in costs.

In what way? Basement and house should be "standard" so far. Do you mean additional permanent reinforcements of the slope at the basement wall or downward?

We have placed the new wall in front of the old one.

The static calculations can be more moderate on your side; we had to support the entire slope. The plot is accessible from below on our side.
Reinforcement, foundation slab, retaining wall, and ceiling of basement/ground floor were made thicker. That was an increase of about 25% on my part.

Demolition costs: check your offer to see up to which hazardous material class is included. Find out what the disposal of the worst class costs and where the landfill is. It varies from state to state.

Earthworks:
1. Question marks: are the demolished houses on the same level?
2. How much further must be excavated additionally, for example, due to modern construction? The foundation slab is thicker, ceiling height, etc. Door threshold height is fixed.
We incurred about 20k extra for earthworks/concrete works on top.

Include the rough work for the exterior area in your planning.

Great tips, thank you very much. Unfortunately, I have few answers to the questions, but it shows me my unknowns.

Much of it you cannot know beforehand.
Pollutant class: all the debris is piled up and samples are taken from it. The worst value determines the class. Especially in the 40s/50s, all sorts of things were baked into stone.

Ground level only becomes apparent once it is gone.