Originally Posted by polly_x

Ive just finished a big conversion on our house so been through this. As D7AKE says your best to take advice as you go along. We used a mixture of the Kingspan foam board and thinsulex foil sheet to achieve the correct insulation rating and save on space. I may be wrong (D7AKE can confirm) but each council can still have different products on their approved list so make sure you check that too. I know of the 3 leading foil insulation products, my area (east Herts) only had one on their approved list.

I ensured that as we were building, we had the inpsector round to see how it was being put together at each stage. That way he had no pulling stuff apart to prove at the end and made it all plane sailing. It also meant he gave guidance on the way which helped.

Not sure about the ceilings. We never had that issue. But you will need to consider fire escape as the rules changed last year. We ended up having to fit fire doors with 1 inch stops on every habitable room in the house to pass the building regs in the end. At £50 a door, plus the fitting, new stops and painting it can easily add a grand to your price to put 5 or 6 new doors on.

In summary, my advice would be to get building control to sign off your design before you start and then keep them involved. That way you should not get tripped up aling the way.