Legal Question in Real Estate Law in California

Actually you have it backward. If the tree roots were growing over onto the neighbor's property they would have the right to cut back the roots but you would have no obligation. When their line is on your property with permission, however, it is either a license or an easement, both of which carry a duty on your part not to interfere with. On the other hand the owner of a license or easement is responsible for maintenance. The two of you could argue whether this is a maintenance issue or an interference issue, but it would probably be better just to share the cost of dealing with the roots.

In the usual case, the tree root (or branch) is the owner's responsibility because it is a trespass and sometimes also a private nuisance. In your special situation, the trespass theory doesn't work, because it's your own land, and I have doubts about whether nuisance is applicable either.

What I do see here is that the neighbor may have an easement for the drainpipe, or it may be on your property under a license. The differences between an easement and a license are usually pretty distinct, but there can be a gray border area where it is uncertain which it is. The proper classification may be critical here, because easements generally aren't revocable and the easement holder has a right to interference-free use of the easement for the purposes for which it was created. On the other hand, licenses are a lesser interest and generally can be revoked - and indeed may have been revoked by operation of law when the licensor (the previous owner) sold to you. If it is or was a license, you have no responsibility to the neighbor.

Factors tending to point to an easement would include (a) an agreement in writing; (b) a valuable consideration; and (c) intention of the parties as to the duration and nature of the use to be made of the affected property - a long use period and a substantial investment in improvements tneding to point to intent to create an easement.

Also, since some licenses can become irrevocable when revocation would be grossly unfair to the licensee, it is probably wise th treat the neighbor's rights as those of an easement holder, unless it is pretty clear the original deal was understood by both parties to be a revocable license. Letting a tree root invade and block the drainpipe would probably be held to be an impermissible interference with the use of the easement (or irrevocable license), and the property owner (you) would probably be held liable. On the other hand, the owner of an easement (the neighbor) is responsible for the maintenance of the easement and the facilities installed to improve and use the easement, so if the pipe itself is failing due to rust, becoming disconnected, etc., the neighbor might be responsible for some of the cost. Arguably, roots do not penetrate well-designed and well-maintained drainpipes.

So, this is kind of a weasel-worded answer, and I hope the two of you can negotiate a realistic settlement, maybe sharing the cost. A lawyer who knew the history of the pipe agreement could perhaps give a better answer, i.e., because it might be clear that this was an easement, or that it was only a revocable license.

Probably. It is likely that the neighbor has an express or implied easement on your land, which you cannot interfere with his use of the easement. While you may or may not have responsiblity to trim the tree root yourself, the least if the neighbor wants to come on to your land to trim it you have to allow.