I spoke to our local planners recently about putting up a polytunnel and they told me they consider them to be the same as a greenhouse, so same regulations apply about size and height.
A while back they tried to tell me we needed planning for an existing caravan till I produced photos showing there'd been one on the same spot for 30yrs+, at which point they backed down.
A lot depends upon who you happen to get on the phone and the way you approach them.