So offer 1:
Compact 30cm of gravel, then single grain concrete, then glue the slab on top. The whole thing with a slope.
The house wall will be further insulated with Styrodur and dimpled membrane, and in front of that, Bitulit, I think it’s called. And a small strip will be painted anthracite along the wall. The cost will be 6000 euros plus slabs 2000 euros, so a total of 8000 euros. All very reasonable, I think, rather cheap. Almost 80m2 will be laid.