Why do I need planning permission?
Planning permission makes it possible for the government to plan for development, and make decisions that will benefit the neighborhood. If everyone is allowed to develop properties without some form of regulation, the result will be chaos.
When your development project involves building a new house from scratch or making alterations to an existing one, the work has to be done in a way that will not affect the plan of the city or cause discomfort to your neighbors. For this reason, planning permission is required.
If you want to add an extension to your house, the local planning authority has to first confirm that this extension is in line with the plans for the city and will in no way affect distribution of amenities, destroy nature or damage the design of the neighborhood. However, planning permission is usually needed for major development work. Smaller projects like loft conversions are considered permitted development and do not need planning permission.