Not quite sure of the planning on sheds, but I thought that providing it is not a permanent fixture like with footing and foundations there was not planning required. On the aspect of roof covering, take a look at cedar wood shingles.
I did exactly the same thing a couple of years ago with a cabin, although wanted to use cedar shakes rather than tiles. The construction was to use a standard boarding arrangement, felt on top of that (which would have been the normal felt roof), then to batten and attach the shakes to that.
I chose a cabin building with low roof pitch so that I could get reasonable floor area without needing to go too high. As you probably know, their are criteria for area, height, distance from house, proportion of plot use etc.
I talked to both the local building control and planning people about it and each asked about their relevant set of parameters. The main one was use (i.e. was I going to use it as habitable space.). I'd done my homework and had made sure that I was within all of the rules, so was prepared with answers for all their questions.
I specifically asked about the roof, and it didn't make a difference.
Great. Its for tool storage and garden machinery, but since the one 100 yards down the garden got jemmied open by some chavs who came across the fields, I want one right near the house where it has to conform to SWMBOCO
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