Shhhhhhhhhh - don't tell the planners !
Mine is going up (without PP and without telling anyone official) it's not huge only 15 x 30 feet so hopefully it'll be okay. But it IS a temporary structure, and I think as long as that's the case it's okay (even if it is dug in, but not concreted)
This is from Premier Polytunnels website - hopefully it'll help.
Do I need planning permission for a polytunnel?
Normally you do not need planning permission for a domestic size/garden polytunnel. However, the permitted development regulations (the rules for what does and does not need planning permission) changed in October 2008, therefore it is always best that you contact your local planning authority if you have any doubts.
If the polytunnel meets with any of the following criteria, you should contact your local planning department to determine if planning permission is required (it will not be required in all cases):
•If the polytunnel would have any part forward of the front of the house.
•If the polytunnel would be more than 3m in height.
•If the polytunnel would be within 2m of the boundary of the site and would be more than 2.5m in height.
•If the polytunnel will result in more than 50% of the area around the house being covered by buildings or structures that were not part of the original house.
•If the site on which you intend to construct the polytunnel has a listed building or is in a Conservation Area, a National Park, an Area of Outstanding Natural Beauty, or the Norfolk or Suffolk Broads – There are further restrictions to planning within these areas.
•If the polytunnel would be for commercial use or any other use that is not "incidental to the enjoyment of the dwellinghouse."
If you want to build a large commercial polytunnel or more than one tunnel, or if you have any queries or questions, please contact your local planning department for further advice.