Making kitchen bigger, can anyone explain this £1.5k per metre rule?

[Kitsmummy]

We have a kitchen that's 16' x 12' and the utility that sits behind it is around 6' deep. We would like to extend out to the side another 8' which will make both the utility and the kitchen 8' deeper (eg kitchen will then be 16' x 20', and inc the utility the full extension would be 22' x 8' (with a wall separating kitchen and utility)

It's single story, currently flat roofed which we would like to change the whole roof to a sloped roof.

So I understand extension costs are approx £1.5k per metre sq, so for us is that just £1.5k per m sq for the additional 22' x 8', but what about the whole new, big roof? Or is it £1.5k m sq for the whole resulting size of the kitchen utility (massive)?

Thanks

Its only a guide, you then have to add in any exceptions / variables etc. The build cost (based on the extra space which is about £24k based on 16 square metres) and then you'd have to cost the extras, like roof, new kitchen. But as I said, I think its only to give you a very rough idea before you get builders estimates etc

Thanks Mandy, that's helpful