This isn’t just a CA insurance law – it’s pretty state-standard all around. Bottom line that even if you send that certified letter, you’re insurance will be covering any damage done by a tree-fall on your property. The only caveat is that you “might” have a claim on the other homeowner as to the cost of removal of said tree. But it’s tough as some insurance co’s require you to hire an independent arborist with a report BEFORE you send the certified letter. Been there, done that.

Unfortunately they are not liable unless you can prove they knew the tree was not healthy and they ignored it. If you send them a registered letter you can show they were informed of the condition of the tree and if they take no action then your insurance company might go after them for damages. but if no damage is done you are responsible for removing the tree from your property! I learned that the hard way when a neighbors tree fell on my house and did minor damage which my insurance company paid for. They paid to remove the tree but I was told that if no damage had been done to the house they would not pay to remove the tree. My insurance went up for filing a claim. What a system but it is Calif law they follow.