With us, the building authority is very picky to ensure that slopes over 1m height difference are constructed at a ratio of 1:1 (45°) and covered with foil.
So according to the civil engineer and landscape gardener everything is okay so far and can be corrected after the house construction. In about 3 months the civil engineer will come again and I will ask him to correct it or something then. We still want to build the wall in [2019]. I think the thing with the wooden piles and so is a good idea.
You see, I didn’t want to encourage more with my comment, but here you immediately get reprimanded about being a schoolmaster and such. Just talk to the civil engineer and have the slope "softened." That’s probably enough until the wall is built. On one side of our house, the slope has been like that for 2 years without anything happening (except for a few crumbs, of course).
Then you have very hard soil I have cohesive, but very clayey soil here that after one winter created the 1:1 slope by itself, ergo it slipped over a large area.
Regarding the wall: large-format sandstone with an edge size of about 1x0.75m set costs about 120 euros per sqm of visible surface here with stones from the local area.
But that is the absolute minimum, due to the large stones and basically no transport distance.