The law generally provides that these limbs (or roots in some cases) can be reasonably trimmed back to the property line so as to avoid danger or damage to you; however, recent legal decisions, in California especially, have held that a property owner cannot “unreasonably” damage the health or even the beauty of a tree by pruning it back to the property line. Obviously this can be a bit of a judgment call. The law is becoming increasingly protective of trees, particularly in urban areas. Property owners who damage a tree that appears not to be causing them any harm at all could have to pay for the damage. It’s best and safest to work something out with your neighbor if at all possible. If danger exists from the tree, you might also contact city officials and get them to back you up.

See also…

Neighbor Law

Real Estate and Property Law