Those saying a person can trim "whatever" hangs over from a neighbor's tree onto their side of the property line are incorrect...yes, trimming can be done but not to the extent where it will/may damage the tree and/or kill it. You can't, for example trim everything off straight up and leave 1/2 of a tree standing....this would most definitely cause the weakening and eventual death of the tree:
https://realestate.findlaw.com/neigh...neighbors.html
"If my neighbor's tree branches hang over my yard, can I trim them?
Yes. By law, you have the right to trim branches and limbs that extend past the property line. However, the law only allows tree trimming and tree cutting up to the property line. You may not go onto the neighbor's property or destroy the tree. If you do harm the tree, you could be found liable for up to three times the value of the tree. Most trees have a replacement value of between $500 and $2500. Ornamental or landmark trees can have a value of between $20,000 and $60,000.
As a general rule a property owner who trims an encroaching tree belonging to a neighbor can trim only up to the boundary line and must obtain permission to enter the tree owner's property, unless the limbs threaten to cause imminent and grave harm. A property owner cannot cut the entire tree down and cannot destroy the structural integrity or the cosmetic symmetry and appeal of a tree by improper trimming."
So, in most cases, overhanging branches from a large tree can be trimmed back to the property line if doing so will not harm the tree or drastically alter its shape/growth but if the main tree trunk was close to the property line (so called "boundary trees") and 1/3 or 1/2 of the tree was hanging over, you couldn't just cut it all off - you would be held liable if you cut everything back and it died (which could be a planned, indirect desire of the person cutting it back). The best case would be to hire a tree service and let them trim it....you could at least then claim a professional service did it and you did not intentionally over-trim the tree leading to its death.
Example and more info:
https://www.tiptoparborists.com/tree...bors-branches/