@Paul
You seem to be wanting to go down the permitted development route, why not investigate planning permission? When I built my games room I HAD to go down the planning route, it really was no trouble whatsoever, benefit was the finished build was exactly what I wanted, well, pretty much... The 'architect' suggested applying for an eaves height slightly higher than I wanted, consequently the ridge was also higher, the reason was if someone was to complain then the height could be reduced and and all parties would be happy, as it turned out there were no objections. So finished build was even better than my original vision.
The planning incurred some costs, but it really was pretty much zero hassle.
Chris,