Floor construction in old buildings, built in 1935 – questions about concrete slab & sealing

Hi everyone,

I am currently in the middle of renovating the living area of an old farmhouse from 1935. The living section on the ground floor has been completely gutted and is being rebuilt. The old concrete floor is gone, currently only the dry, solid sandy ground is exposed.

The exterior walls have already been newly insulated and clad with bricks (two-shell construction: 11.5 cm sand-lime brick + 20 cm insulation + air layer + 11.5 cm brick).
Now it’s time for the new floor construction – more precisely, the concrete slab. The following points are currently open for me:

[*]Clean layer or foil?
Some say PE foil directly on the sand is enough. Others recommend a thin concrete layer (5 cm) to create a clean surface. How did you solve this?
[*]Sealing under or over the concrete?
The old horizontal barrier in the masonry is about 20 cm above the future concrete floor. Now I ask myself:


[*]Is sealing under the concrete sufficient?
[*]Or should I additionally seal again over the concrete (under insulation)?


[*]How thick should the concrete be?
I do not plan any load-bearing walls on the slab. My feeling is that approx. 16 cm C25/30 should be enough. Or should I go for 18–20 cm after all?
[*]Letting concrete run into the walls – yes or no?
One contractor advises cutting a 20–30 cm wide slit in the KS walls every 80 cm so the concrete can bond in. Another says that weakens the wall unnecessarily. I tend not to do it, since the sand-lime brick wall has already been standing on a foundation for 90 years, the "concrete fingers" in the sand-lime brick wall could also be potential weaknesses, right?
What do you think? Has anyone done this?

I look forward to your experiences or tips. I’m open to anything – gladly also with example constructions or photos.

Best regards
Ole