Albags is quite correct. Legalities differ from council to council. Another alternative to
going to see them personally, is to look on your councils website, they should give all the necessary criteria.
AIBags and ifunewsusie are quite INCORRECT. The link I have posted is to the government website and shows the national permitted development rights for England. These are the developments that can be carried out without planning permission. The only exception is if there is an 'Article 4' direction on your property or if there is a condition attached to any previous planning permission at your property removing permitted development rights. The policies that differ between local authorities are for developments that DO need planning permission - which is not what you asked for. I'm a planning officer so I know what I'm talking about!