Sorry if I gave out wrong information, my experience was in the Cotswolds, looking up the current rules I see;
//However you will need to apply for planning permission for some cases, for example:
•outbuildings and other additions exceed 50% of the total area of land around the original house. Sheds and all other outbuildings and extensions to the original house must be included when calculating this 50% limit
•the buildings are forward of the principal elevation of the original house
•the building is more than single storey, or its eaves exceed 2.5 metres, or it exceeds 4 metres at ridge height where the building has a dual pitched roof, or exceeds 3 metres at ridge height in any other case
•the new building itself is a separate, self contained, living accommodation
•the building is more than 2.5 metres in height and it is within 2 metres of the property boundary//
The last sentence seems to apply here, but I don't know how universal this is.