Do you need planning permission?
In general you do not need planning permission for sheds, greenhouses, summerhouses or polytunnels. There may be some restrictions if you are in a national park, conservation area or area of outstanding natural beauty. So the answer is generally "no", at least for domestic tunnels (not large commercial ventures).
Limitations on size
The ordinary allotment tunnel should not be more than 3m high (or more than 4.2m at the ridge) or more than about 18m long and about 4.5 m wide at base in order to avoid planning permission problems.
Planning disputes
Here is some ammunition if ever you need it (if not, skip to next section):-
Discussion
Development of land generally needs planning permission: but Section 55(e) of the Town & Country Planning Acts 1990 states that: "the use of land for the purpose of agriculture or forestry and the use of any building occupied together with the land so used" does not constitute "development" so does not need planning permission. Allotments of course are agricultural land; and converting land to agricultural use (e.g. by turning it into allotments) does not require planning permission either.
The Royal Institute of Chartered Surveyors summed this up: "Structures may be placed on agricultural land for agricultural purposes without the necessity for obtaining planning permission. However, no building or engineering works must be involved." OK to place the polytunnels on the land but not to pour a cement floor unless you can remove it when you leave.
There was a court ruling that "Spanish Polytunnels" - large, walk-in, double-spanned plastic tunnels - are a development operation and so required planning permission (Hall Hunter case December 2006). Some Councils then imposed new charges and restrictions on small polytunnels as well, until Paul Hudson, the Communities and Local Government Chief Planner, sent round a letter to stop this.....
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