It became a lot easier from 1st October 2008 and now many of these type of projects can be done without Planning Consent - they are regarded as 'permitted development. The new constraints that apply now are (edited to include only the relevant ones):
•No outbuilding on land forward of a wall forming the principal elevation.
•Single storey, with maximum eaves height of 2.5m and maximum overall height of 3m.
•Maximum overall height of 2.5m in the case of outbuilding within 2m of a boundary of the curtilage of the dwellinghouse.
•In 'special areas' (National Parks, the Broads, AONB and World Heritage Sites) the maximum area to be covered by buildings more than 20m from dwelling-house is limited to 10 sqm.
•If the dwelling-house is listed, any outbuildings will require planning permission.