i wouldn't worry too much about planning permission
this is what it says about sheds.
Under new regulations that came into effect on 1 October 2008 outbuildings are considered to be permitted development, not needing planning permission, subject to the following limits and conditions:
No outbuilding on land forward of a wall forming the principal elevation.
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 2.5 metres within two metres of a boundary.
No verandas, balconies or raised platforms.
No more than half the area of land around the "original house"* would be covered by additions or other buildings.
In National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites the maximum area to be covered by buildings, enclosures, containers and pools more than 20 metres from house to be limited to 10 square metres.
On designated land* buildings, enclosures, containers and pools at the side of properties will require planning permission.
Within the curtilage of listed buildings any outbuilding will require planning permission.
when we built ours we just checked the neighbours sheds and garages and they all seemed to be right on the boundary