For most new structures you will be required to gain permission from your local authority before carrying out any work. This involves submitting an application with drawings to the planning department of your borough council. They will then consider your proposal, making an assessment of the impact on surrounding residents, building architecture and environment. The council will have a set of guide rules for the type of works that they are currently approving in your borough. These are assessed and applied on an individual basis. Your neighbours will also be consulted to see if they have any objections or concerns.
Planning permission can be gained retrospectively although there is always the risk that if the application is not approved you will have to take down or alter the existing work you have had done.
Costs for a householder application is £150 and most councils aim to have a decision made in 8 weeks.
Many Local Borough Planning departments offer a walk-in service where you are able to meet with a planning officer to discuss outline proposals. This can be useful in gauging current policy and of how successful your application may be.
For some types and sizes of extensions and loft conversions full planning applications are not required. These works can sometimes be carried out under the local councils permitted development guidelines.