Subjects - Your home - Building regulations
If you want to do more or less any sort of building work in this country subject to very minor exceptions you have to get the permission of the council under the building regulations.
The building regulations are designed to ensure that when you build something it stays a sturdy building that isn't something that's going to blow over the first time the wind blows, and is healthy and safe and warm and energy efficient and so on.
That all sounds very good in theory. In practice however it can develop into a bureaucratic nightmare and the building regulations themselves and the rules around them can be very complicated, so you might need expert advice from a lawyer.