Legal Question in Real Estate Law in California

Re: property line

If part of the tree trunk is on each side of the property line, the tree is jointly owned, and neither property owner can cut it down without permission of the other. If only roots or limbs cross the property line, that's another story; the tree belongs to the neighbor on whose property the trunk stands, and the roots or limbs crossing the property line constitute trespasses by the tree owner. The victim of the trespass has a remedy: to cut the limbs or roots at the property line. There is an important limitation on this remedy: the trimming must be done in a non-negligent manner, which probably means in a way that will not kill the tree or make it prone to topple in a windstorm.

Your question indicates that the size of the tree would now require repositioning the fence onto your property. This suggests that the trunk is partly on your property now.

So, it looks to me that, absent an agreement, the replacement fence will have to take into account the existence, joint ownership and position of the tree. The fence should be designed and built to accommodate the tree, not deny its existence. This might be handled by attaching the northerly side of the fence to the north side of the tree, as if it were a fence post, and then the southerly extension of the fence to the south face of the tree.

Boundary fences don't necessarily have to be precisely on the property line, by the way. Wiggly and inaccurate fences rarely if ever result in shifing any ownership rights of an owner to the neighbor in urban settings.

Re: property line

You can cut away any portion of the tree encroaching you your property line.