Most outbuildings such as garden offices, garden sheds and garages do not need planning permission so long as it is built on your own land and comply with the regulations below:

• Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof.
• Maximum height of 2.5 metres in the case of a building, enclosure or container within two metres of a boundary of the curtilage of the dwelling house.
• No verandas, balconies or raised platforms (a platform must not exceed 0.3 metres in height)
• No more than half the area of land around the “original house” would be covered by additions or other buildings.
• No outbuilding on land forward of a wall forming the principal elevation.
• On designated land* buildings, enclosures, containers and pools at the side of properties will require planning permission.

A notable exception to the above information, is when you wish to build an additional building within the grounds of a Listed Building. What is not well known is that everything within the grounds of a Listed Building is listed, not just the house/wall/structure which is of historical interest. So whatever you build becomes part of the listed building in the future and therefore needs planning permission.