Insulate floor slab - ceiling height is not sufficient

Well, that's all rather pointless cosmetics when there's 15 cm of topsoil underneath.

Well, that's all rather pointless cosmetics if there's 15 cm of earth beneath.

Why? The concrete floor is just a floor - remove it and excavate a bit to install insulation and a modern floor structure. Whether all of this is economically sensible for a house from the 1940s with technology and equipment from the 1940s is something everyone must decide for themselves. Based on intuition and experience (technology, ceiling heights, cuts, etc.), I recommend the big excavator and a nice new build.

because without a proper, capillary-breaking layer, there will always be problems with moisture? A foil under the slab is not enough.

Also think that concrete was simply poured in there. Either do nothing or remove the floor and replace it. I would have reservations with anything else. We saw during the demolition that where modern materials met old systems, there was mold, dry rot, and the like.

Whether all of this makes economic sense for a house from the 1940s with technology and equipment from the 1940s is something everyone has to decide for themselves. From gut feeling and experience (technology, ceiling heights, layouts, etc.) I recommend the big excavator and a nice new build.

I would like to, but unfortunately it’s not possible, since there is a separately inhabited annex from 1989 that would then collapse. So either modernize the existing as best as possible. Or don’t buy at all.

Either modernize the existing one as well as possible. Or just don't buy it.

Do the latter. As well as possible is similar to nice...