a minefield. Approved Document L has expanded from three sides of a fag packet to 1,000,000 pages of crazy and irrelevant calculations that nobody understands. That's his big problem, but, hard on the heels of that you'll find Approved Document A, C, F, N and a couple more besides...........
He should do one of two things. Either go and see his local Building Control Department, who may have a simplified policy on this (remember that the Approved Documents are guidance and Building Control has the power to decide) or go and see an Approved Inspector who has a clear approach to this. And then he should find himself somebody capable of designing the job. And that's not me, because even if I was so hard up that I really needed the money, these small domestic jobs are a recipe for heartache and pain.
And if he's got a mortgage he needs to inform the building society or bank, and write a letter to the planning department seeking advice.
(later edit) and you, Mr. Jogger, should swear to yourself that you will not, under any circumstances, get involved.