Home DIY

if it's under 2.5 meters tall then you shouldn't need planning, unless you put a satelite dish on it. Likewise as it's not a habitable space, as in it's not in constant use, then you shouldn't even need building regulation approval. Although if you run any electrical cables or put in extra drainage etc then you will.

http://www.thegardenroomguide.co.uk/building-regulations-for-garden-rooms/2012/08/18/
"You don’t need Building Regulations for a garden room if:

The internal area is less than 15m2
If the Internal area is less than 30m2 and is not within 1 metre of the boundary"