Planning permission is required if the shed is placed in your front garden (unless you've got such a long front garden that the shed would be at least from the highway).
Planning permission is also required if the shed is over 3m high (or 4m if it has a ridged roof).
Planning permission is required if the volume of the shed exceeds 10 cubic metres
and you live in a listed building, conservation area, National Park, designated area of outstanding natural beauty or the Broads.
Planning permission is required if the shed (or a combination of the shed and pre-existing outbuildings) will occupy more than 50% of the land surrounding the original property.
Planning permission is required if the shed is to be used for 'non-domestic' purposes.
You also need to check the deeds of the house for any relevant covenants.
Otherwise, you can probably erect your shed / summer house without needing planning permission:
http://www.communities.gov.uk/pub/445/Planning AGuideforHouseholders_id1500445.pdf20m
Chris