"Christian McArdle" wrote in message
et...
25 or 30mm foam underneath

From memory, don't you need 50mm underneath? You need around 100mm
Kingspan/Celotex to get to loft conversion standards, don't you?

Yes and no. There is a get-out clause which allows a U of 0.3 for
conversions. But I'd still agree your proposed way is better.