Do you need to apply?
You need planning permission for most building works or changes of use of land or buildings. Planning law defines 'development' as :-
'the carrying out of building, engineering, mining or other operations in, on, over or under land, or the making of any material change in the use of any buildings or other land.'
However not all building works or changes of use require the submission of a planning application to the Council. There is a wide range of development which is granted planning permission by regulations made by central government.
If you are in any doubt whatsoever about whether you are required to apply, you should consult the planning department of your council. They will normally give you advice, but if you want to obtain a formal ruling you (or your adviser) can apply(upon payment of a fee) for a "lawful development certificate". This can be done by writing to the council with details of the work you want to carry out.
Beware! If you build something that requires planning permission without obtaining permission first, you may be forced to put things right later, which could prove costly, inconvenient and troublesome. You may even be required to remove an unauthorised building.