UK regulations concerning external staircases
In the U.S., our Codes require a “landing” at the top of stairs that are as wide as the stairs and extends the width of the stairs...so it’s a landing that’s as wide and as deep as the width of the stairs. 

So, that’s a minimum dimension required. If the previous owner enlarged it, you’re lucky, especially if you’re going to add a door that swings into that landing. Then minimum dimensions change, especially if it on a side next to the top step.