Foundation Structure

The green represents insulation. A rigid board type. And the red line above that appears to be a screed (floor finish similar to concrete). Its a little hard to make out why there is a space under the insulation.

Insulation like that should be around 50mm thick, but depending on various factors such as Thermal resistance wantd/required, and the quaility of the insulation used, the thickness can vary a bit.
As for the screed, like the one shown (floating screed) should be around 65-75mm thick.

As for the cost, It can vary alot depending quaility of materials, workmanship. I wouldn’t even hazard a guess to be honest. What sort of job is it?
The Homebond book would be a good buy to help you.

Often wondered Bren88 about what you mention there. Presumably the insulation is polystyrene slab or equivalant branded product, in which case how does the screed remain stable and intact on such a comparatively soft and moveable surface?
Could be comepletely off the mark here in how things are done…

For insulation in this case, it’s placed on the sub-floor slab and the insulation itself is specified to have a minimum density of 20kg/m2 and be grade ‘HD’ in accordance with BS 3837: Part 1:1986.

The practical upshot of which is that once the correct insulation is used, it won’t be soft & moveable.

The screed would also be a minimum of 65mm, preferably 75mm thick and be reinforced with 1 layer of A193 mesh.

Question was pretty much covered by Spanky.
As long as the rigid board is dense enough it will hold. Because the screed has less support its thickness is increased. It’s fairly standard practice.

Aha – thanks both for that. A long-held question answered 🙂
No doubt the mesh plays a particularly important role.

Yeah the mesh is important, If the screed is thick enough it is not needed everywhere, but its is always wise to use it over pipe and services in the screed.