It may also be worth checking whether there are any restrictive covenants on the land registry title to that property which may benefit adjoining owners. It is not uncommon for there to be some form of restriction on building in a garden, with the benefit of that restriction going to the neighbours.

The planners aren't concerned with that kind of thing as it's not material to them but it gives the neighbours a great big legal stick to beat the developer with.

You would hope that the developer's solicitors would have warned him of such a restriction but it's a pretty big area for solicitors' professional negligence so I wouldn't bet on it...