In England and Wales you will not usually require planning permission, provided you do not exceed the permitted development limit for your property (which is 50-70 cubic metres, depending on where you live).
Exceptions to the above are listed buildings and conservation areas.
If you have any doubts, we advise you to check with your local planning department.
Click on the links below for more help.
South Oxfordshire District Council Planning
Vale of the White Horse District Council Planning