“Permitted development” legislation permits your building with no planning formalities. Or an optional simple one. It defines various limits and requirements, such as height and exterior appearance of materials. Read the legislation in detail here.
Architects interpret this legislation as far as it applies to your extension. If your design is within the limits you do not have to make an application for Planning approval. You can, but don’t have to, apply for an optional Lawful Development Certificate from the Council. That confirms your design is Permitted Development.
