In 2013 the government made changes to planning permission, allowing single storey home extensions to be built with no permission as long as the size of the proposed extension met current guidelines.
If you require a formal decision as to whether planning permission is required you can submit a certificate of lawfulness.
The publication guide for householders sets out the rules concerning what extensions, improvements and alterations a householder may make to their house and the area around it without the need for an application for planning permission. Such works are known as permitted development.
For more general advice regarding planning permission for householder developments, there are a number of interactive guides on the planning portal.