Technically, yes PP is required (you are not going to move your storage shed every few months or so to get away with temporary-structure status are you?). But then many folk get away with very minor, unobtrusive "developments" without any problems at all.
When I was going through my planning application the planning officer was not persuaded that my proposed ISO containers might be positioned quite so close to the adjacent road and then he used a neighbour's stables construction as an example of a more acceptable rural development. I didn't tell him, obviously, that his good example of rural development didn't actually have the PP that it needed (under the rules) !!