Make up beneath suspended floor slab

[samtheplan]

I am constructing an extension with a suspended concrete floor.

Why is there a requirement for 150mm hardcore beneath the blinding and dpm?? If the slab is being suspended from the founds and has mesh etc in it? does it matter what the ground is (as long as its not clay (heave)) after all it is just acting as a permanent formwork??

I amt trying to avoid digging out and refilling as the existing level is almost the same as the bottom of slab level.


Thanks