In general you don't need permission...unless it's going to be a huge monstrosity & you want rent it out to holiday makers! :o)
It might be best to get in touch with your local authority to check so that you you don't get some know-all hanging over the fence saying 'You can't do that...' (there's always one!)
Robinia is right. When I worked at the council I used to take great delight in advising people that you could practically build a ski lodge in the garden without permission. And we had a booklet saying so. In this day and age of overbearing rules and regs its amazing you still can.
As far as I'm aware, so long as the summerhouse is simply resting on the ground, (like many wooden garden sheds), it's classed as being portable, and therefore wouldn't need anything in the way of planning pemission.
But if you're building it on foundations, it becomes a fixed structure and permission would be needed. (Maybe Buildersmate could comment on this).
I don't think so, Heathfield. A caravan is a temporary structure by your proposed definition but you can't just park one in the back garden and start living in it (not without the Planning people showing an interest).
What is true is that it is easier to ignore the 'rules' with a temporary shed or summerhouse because it can always be moved relatively easily if the Planning Authority threaten to slap on an Enforcement Notice.