The whole of a tree belongs to the owners of the land on which it stands. Trees neither recognise or respect boundaries and there is no obligation under common law to cut a hedge or prune a tree growing near the boundary of a property, however much of the neighbouring land owner may object to it.
It is advisable to seek the owner's agreement before carrying out any pruning work, and the effect of work on a tree's health should always be considered. A neighbour is not entitled to reduce the height or prune any part of a tree or hedge which does not trespass over the dividing boundary.
A neighbour is entitled to remove trespassing roots or branches at the point where they cross the dividing boundary, but not beyond. The material removed remains the property of the tree owner and should technically be offered for return, before disposal.
If a tree is included in a Tree Preservation Order or a Conservation Area then no work can be carried out (exceptions may apply) by either party without prior written permission from or the requisite notification being served on the local planning authority.