Very thin screed, earth underneath - what to do?

The walls must have foundations. These should be significantly deeper than you dig. Work directly at the wall with the shovel instead of the excavator.

Of course, you must not undermine the foundations.

Foundations are not present; the walls stand directly on the rammed concrete.

I have now looked at it again carefully. The house, or rather the part to be renovated, is about one meter above the regular ground level. It was therefore built up and bricked up very high until the rammed concrete was poured. There have never been any problems with moisture. Even the fill underneath the concrete is bone dry.
As my parents told me, their house next door is built the same way. That is why there are no cold feet in the deepest winter when walking on the parquet or plastic floor.

Now I am a bit confused. On one hand, a new floor structure would involve considerable additional effort, on the other hand, it seems to work well without it. Sealing the floor with bitumen or otherwise against moisture and then laying a thin screed over it would be quickly done and would only cost 2-4 cm of room height.

What do you think?
Not that I haven't read the many answers, but I am somewhat surprised that the construction at my parents' house causes no problems.

Not that I haven't read the many responses, but I am somewhat surprised that the structure of the parental home does not cause any problems.

There are so many different historical construction methods that one cannot judge everything from a distance. But: the comfort expectations of previous generations are just as different as the renovation materials used today. If the house is heated, ventilated, and insulated as it was 50 or 100 years ago, it can work. But if you start installing underfloor heating and modern windows and doors, vinyl floors, shower and cook more often, it can lead to building damage, especially mold.

Foundations are not present, the walls stand directly on the rammed concrete.

I took another close look at it. The house, or rather the part to be renovated, lies about one meter above the regular soil. So it was built up very high and masonry was done until the rammed concrete was poured. There were never any problems with moisture. Even the fill under the concrete is dust dry.
As my parents told me, their house next door is built the same way. That’s why you don’t get cold feet in the middle of winter when walking over the parquet or plastic floor.

Now I am a bit confused. On one hand, a new floor construction would involve considerable additional effort, on the other hand it seems to work well without it. Sealing the floor against moisture with bitumen or otherwise and then pouring a thin screed on top would be done quickly and would only cost 2-4 cm of room height.

What do you think? Not that I haven’t read many answers, but I am somewhat surprised that the construction at my parents’ house doesn’t cause problems.

One thing is theory and the other is practice. If the sand under the screed is dust dry it insulates relatively well and no moisture problems are expected in the future. I would recommend 4 cm of PUR insulation laminated with aluminum and 4-5 cm of calcium sulfate screed / anhydrite screed. Lay construction foil under the screed and edge insulation strips on the sides (floating). Done. Might take 90 minutes for a 20 m2 room. Then the underfloor heating also has hardly any consumption, provided there is also a radiator in the room in addition to the underfloor heating?

edit: since this is an 'old building' I would get a second opinion from a building expert. Maybe 4 cm cement screed would be better. For bending strength I would then recommend light reinforcement mats in the lower third (tensile stress). This is just my opinion/advice.