I mean I had to ask what kind of building the floor was poured for. Now we know its an extension we can ask:
1) Is there any insulation under the slab?
2) Have you asked your builder?
3) Is the floor finish to be boards or a screed?
4) What depth have you between the top of the concrete and the finished floor level in the existing building?
5) Are you doing this to building regs?
6) What do the drawings say?
Typically you'd expect approx. 90mm of rigid insulation such as Kingspan or Celotex for an extension to a dwelling depending on the perimeter and area of the extension.