at the end of the day, you'll build what you think is best, but have a look on e bay at portable garages/greenhouses, it's just another idea, that could work for you, can be picked up relatively cheap, and as it's portable, cannot be classed as a permanent structure by councils, if they did object to a solid pallet roof type building
I picked ours up for £80, it's changed a bit since we first put it up, we didn't secure it down well enough and the high winds took it away a few times, it's now lower, totally held down, and has a cheap log burner in it (another e bay find)
keeps us dry and warm on the worst days
http://youtu.be/_qdTiKSElME