Yup did this last year bought the bottom half of a long narrow garden so ours is now L shaped.
Had a couple of agents round and gave us very different valuations, but at the end of the day its how much its worth to you and them and your house which depends on the size of house ie family home or not.
They started at 16k i started at 9k we agreed on 12k so we were both happy.
We both paid our own legal costs, but I did the fencing. We now have a nice veg area and firepit with fruit trees and a large Oak tree that I had been itching to have made safer anyway as they weren’t going to do it.
I am now also thinking about a treehouse………. 😉