Planning permission is granted by the planning department of your local council. It is your responsibility to find out if you need planning permission and apply for it before building work goes ahead. If you do not, you may be asked to make a retrospective application, which, if unsuccessful, could result in the work being reversed and the building returned to its original state or new structures being demolished.
Planning permission is not always required. If you do not live in a listed building or conservation area, you can carry out a wide range of construction or alteration work on your property without planning permission, but it is always wise to seek advice to make sure. However, even for alterations to the intertior and small external projects you may require Building Control Approval to ensure building work conforms to Building Regulations.
Work that will require planning permission includes the following:
Every house has a permitted development allowance, which means that you can increase the volume of the house by a set percentage of the volume of the original house. This is the house as it was first built, unless it was built before July 1948 in which case the term "orginal house" refers to how the house was on 1st July of that year.
Living in a listed building or conservation area does not automatically preclude you from carrying out work on your property, but you will need to apply for listed building consent. This may even be required for relatively minor maintenance work such as pointing or painting the front door.
Specific advice will be available from your local council. For a general guide to planning permission visit:
http://www.planningportal.gov.uk/
http://www.directgov.co.uk/