I’m assuming that you have a detached property if you are seeking to build a 8m rear extension under Permitted Development rules. As a precaution, I would always start by checking with your local planning office that your permitted development rights have not been removed. Then, you will need to submit a full set of drawings (plans and elevations) and your neighbours will be consulted on the proposal. There will be the usual considerations of overall height, impact on the surroundings and percentage of the curtilage used for all extensions. Once your application is approved, you may commence construction.