Planning permission is the approval your local authority gives to allow you to build new or change existing buildings. 

Planning permission from your local authority is required for building, rebuilding or demolishing a property, subdividing the property into more than 1 dwelling or mining operations.

An extension is deemed a permitted development, and planning permission is not required as long as the extension does not exceed half of the land around the original house. It is based on when the home was brand new. When you are working this out, please do not include any alterations already undertaken.

The height of your extension matters too.  It cannot exceed the highest point of your roof; otherwise, planning permission is required.

If you were looking to include a balcony in your extension, this would not be permitted development. You would need to seek planning permission for this aspect of the build.