Trees on one person’s lot often grow to overhang the neighboring lot. Real property includes ownership of the airspace above the land as well as the ground under the surface, so the overhanging branches are the property of the neighbor, who, in most cases, has the right to cut off the overhang, or, alternatively, to own any fruit produced by the overhanging branches.
Like branches, roots also can cross into the neighboring lot, and when they do, they are more likely to become a problem for the neighbor than overhanging branches, possibly pushing up patios or sidewalks, or invading sewer pipes, or becoming above-ground or below-ground obstacles to landscaping or gardening.
Do not take any action to prune overhanging branches or invading roots on your lot without first getting your neighbor’s permission to do so, because your “ownership” as described above of the parts of your neighbor’s tree that are on your lot is subject to the universal rules in the law that a person must act reasonably and may not use his rights to harm the rights of another person. If the tree is seriously damaged by action you take without permission, you will be liable for the injury. Discuss the issue with the neighbor and get his agreement to what you propose. Confirm the agreement in a letter and keep a copy of it.
If someone injures or destroys a tree on your land, under California law, you may have a claim for twice the value of the cost to cure the injury or replace the tree – three times the cost if the trespass that caused the injury was willful. Arborists have developed formulae for appraising such injury or replacement values.