I believe that Atheist is Ex-Planning by profession. He is absolutely right.
There are however rules for "Permitted Development". These allow for some extensions that do not need PP.
I've include a link here to help you get an idea, before you talk to the Planners.
Arm yourself with the facts, so you'll be able to relate to what the Planning Officer tells you.
You may well be able to go ahead without troubling them, but, as Atheist says, you must be sure of your situation.
https://interactive.planningportal.co.uk/