I'm planning on replacing my suspended timber ground floors (1920s semi) with insulation-under-slab (bearing onto ground).

The planned build-up is (bottom-to-top) [mumble], sand blinding, DPM, 100mm EPS100, PU foam glue, 100mm PIR (non foil faced) DPM, 65mm Eco Screed (containing steel reinforcing grid, with UFH pipes tied to it).

I'm using the PIR because I have lots and lots of it (bought up a load of factory waste from a place making insulated metal roof and wall sheets ~120kPa compressive strength). DPM will be taped with butyl joint tape, and will probably be a radon-barrier grade (although it's not radon area, I'm hoping this will retard the pentane loss from the PIR somewhat, as well as keeping it dryer long-term). I'll stagger the two insulation board layers, and use a PU foam glue so that it essentially ends up as one big solid block of insulation.

Under the floor, there is currently chalk and flint rubble (it's on the South Downs), which I think has been mixed for the top few inches with lime (or possibly portland cement) to provide a hard over-site layer. I plan to do a couple of small test excavations to ensure that there's no soil lurking underneath that layer.

My builder is under the impression that our BCO will want the top of that dug out, and compacted hardcore put down instead. Seem pretty nuts to me, and both my builder and I would rather avoid that...

I know very little about ground floors and footings, but is it likely to be possible (cost effective?) to get a structural engineer involved, or should I be able to talk the BCO around without that? I'm guessing that some sort of sub-base hardness test will be needed and/or the use of a vibrating plate compactor?

I'm planning to use a perimeter down-stand of 100mm EPS100 too (below DPC) to reduce thermal bridging - will that effect things at all?

Im planning something similar on a job. I have a void under a sudspended floor of 450mm under the boards, The sub base is level [ish] dry soil /ash. There is no building Inspector involved and I have absolutely no intention of using hardcore. I indend to level the surface and blind with sand. Then 250mm of polystyrene, 130mm concrete, 50x50mm treated timbers and floorboard finish. My reason for changing to the slab is that through ventilation cannot be maintained to a sufficient level if the joists are insulated between.

My understanding regarding the use of hardcore under slabs is to provide a surface which will spread the load of the slab while allowing minimal settlement/movement. This principle being formulated long before the levels of insulation required in floors today. I suspect most if not all rigid insulation will serve the same purpose as the hardcore.

Why not try and put this argument to your inspector

Harcores also there to make sure there's no organic material under the slab that may be unstable.
i presume that chalks been there nice and stable for quite a while.
How about attack as a means of defense.
eg. saying something like "the existing well compacted flint/chalk hoggings would be best left along as it'll make an excellent hardcore base for the slab."
then move on swiftly to another area of the project, using the old ego diversion trick "I'd be greatful of your advice on this ,"thing", as I'm not sure of the best solution "