Not totally sure on this, so don't hold me to it, but I think the key here is whether a structure is permanent (ie, brick) or temporary (wooden). If it's temporary, as far as I know there would be no size rules, but if brick, it would be classed as a permanent structure, and maybe be subject to planning applications and restrictions. I know when you are building an extension, you are allowed to add so much without permission but beyond that you need to apply for it. Though it's only a shed, chances are if it's permanent, it would come under those same rules.