Legal Question in Real Estate Law in California

You can cut the tree, up to the property line and without hurting the tree. Otherwise, you can bring a legal action to make your neighbor cut the tree. The neighbor has committed an encroachment. You can sue for the lost value to your property or the cost of cutting back the tree.

Best of all....bring some cookies to your neighbor and ask him or her to cut back the tree.

As the previous answer indicates, you have two choices:

(1) You are entitled to address this particular kind of trespass without going to court, by doing the trimming yourself (to the property line) and at your own expense. If you choose self-help, you must do the trimming in a way that is "non negligent," i.e. does not injure the tree and does not create a windstorm hazard.

(2) Alternatively, you can go to court and sue the neighbor for trespass and/or private nuisance, and presumably get a judgment awarding you money damages and/or an injunction abating the nuisance.