Block and beam build up

You would need to follow modern regulations which would likely require 75mm PIR (Kingspan type insulation) or 125mm Polystyrene covered by a 50-75mm screed. So your new block and beam floor would need to be set lower than your existing.

As far as your block and beam build up is concerned you need to work from your existing finish floor level back, allowing for the necessary make up of structural floor & insulation most suitable for the intended use, ie you say tiles & underfloor heating- we say min 65mm screed on 75mm Celotex as this is more stable for the application than your current floor, so 65mm screed plus 75mm Celotex plus 150mm beam equals 290mm from FFL to you first dpc level or underside of beam, you can add to the screed thickness to make this fall inline with the existing brick courses on your house Your existing floor is commonly known as a floating floor or a warm floor, if you are intending to continue the tiling into the existing property you maybe wise to remove the chipboard and polystyrene from the proposed area to be tiled & screed this (inc new slimline high performance celotex ) along with the new floor in the extension as screed floors are much more suitable for the application of tiles Best regards Hartwell Projects ltd