Legal Question in Real Estate Law in California

There are several possible approaches, and the best one in your case may depend upon local conditions, both as to the trees themselves and the neighbor's degree of recalcitrance. First, I'd start by hiring a licensed arborist to give you a couple of written estimates including professional commentary on the condition of the trees -- one for trimming back to the property like, the other for removal. Next, confront the neighbor with the written expert opinion and the estimates, Assuming there's still no cooperation, you can (a) have the trees professionally trimmed back to the property line and send the bill to the neighbor, which may require enforcement in court, but at least you'll be (presumably) in small-claims court; or (b) you can sue for trespass (or "treespass") since the overhanging limbs and invasive roots are a form of trespass to land, and you can also ask for damages for the cost of repairs to your wall (this will require another contractor estimate as substantiating evidence). I somewhat prefer method (a) because the suit can be in small-claims court,