Solid concrete floor insulation

[Dan77]

whilst preparing the over site for my extension I've come across an existing R/C slab 240 mm below my new FFL.This only allows for 75mm of insulation(100mm concrete,75mm insulation,65mm screed)does anyone know if this will be acceptable to BC?
My building inspector only works Wednesday to Friday and I'd like to get it prepped this weekend and I don't really want to break out the slab which is 300mm thick!

 

[John D v2.0]

If you have a 300mm reinforced concrete slab and can convince the building inspector it's true, then all you need is a little sand, dpc and your insulation and screed.

 

[JohnD]

how old is the house?

You may find the original plans on the councils Planning Permission and Building Control pages of their website.

 

[Dan77]

House is 1950s
The slab only covers about half the area of the extension so I've dug round it, put in 150 of hardcore topped off with a little type 1 up to the same level as the slab and was hoping to sand blind it polythene and 100 mm concrete with mesh to strengthen against cracking but this will only leave 130 to 140mm for insulation and screed.dya reckon that'll be okay?

 

[John D v2.0]

Insulate first then slab then self leveling? Or perimeter trench insulation instead of underneath? That would give insane levels of thermal mass though.
Or slab then some kind of framework with insulation between, but you couldn't use timber because you'd struggle to ventilate it.
Basically you need to lose one of your layers from the equation, if you can't lose the slab you need to lose another.
I've never done this personally and don't leave your design to randoms online!