Good q Lurker.
We're having it done becasue that flat roof was 20 years old an on its last legs. We didn't realize how far gone it was until they pulled it off, it practically fell down, we're lucky we've not had water ingress before.
So the options were to spend money putting on another flat roof, which would still look like a carbuncle, run a slope across between the two side rooves (but they are not quite the same slope so something would have to give, and it would mean changing the bedroom window anyway) or rebuild. And as we'd had the Water episode last summer, and the life of a roof like ours is about the age it is now, we decided to bite the bullet.
Yes it would have been nice to extend the workshop but that would mean taking up more of the garden and i don't think that that would have got past management. Anyway, this is taking all our money as it is (well, all J's money to be more accurate), an even bigger project would be out of the question.
BTW, I'm quite pleased with the way they are doing the job, so far so good. I'll be glad when it's done though.
Cheers
Steve