California law states the presumption that two neighbors benefit equally from a boundary line fence and that they are therefore equally responsible for it. Of course if you want to be on good terms with your neighbor you would discuss a fence repair or replacement before beginning the work. The law specifies certain obligations which you should understand.
The original law defining a property owner's rights and obligations for property line fences was created in 1872 but updated in 2013 and is cited as the "Good Neighbor Fence Act of 2013". The key parts of the act are:
(1) Adjoining landowners are presumed to share an equal benefit from any fence dividing their properties and, unless otherwise agreed to by the parties in a written agreement, shall be presumed to be equally responsible for the reasonable costs of construction, maintenance, or necessary replacement of the fence.
(2) Where a landowner intends to incur costs for a fence described in paragraph (1), the landowner shall give 30 days’ prior written notice to each affected adjoining landowner. The notice shall include notification of the presumption of equal responsibility for the reasonable costs of construction, maintenance, or necessary replacement of the fence. The notice shall include a description of the nature of the problem facing the shared fence, the proposed solution for addressing the problem, the estimated construction or maintenance costs involved to address the problem, the proposed cost sharing approach, and the proposed timeline for getting the problem addressed.
A few exceptions to the Good Neighbor Fence law:
What if one of the neighbors says they don't care if the fence falls down? Generic advice published by attorneys is that probably the court would find that he benefits from the fence and hold the neighbor responsible for half of the cost as long as the specified conditions were met.
California Civil Code Section 841.4 puts restrictions on fences which are over 10 ft. tall and are often referred to as spite fences. One of the important aspects is that the fence does not have to be made out of typical building materials. A tight row of trees obstructing a view could fall into this category.
California Civil Code §841.4
Any fence or other structure in the nature of a fence unnecessarily exceeding 10 feet in height maliciously erected or maintained for the purpose of annoying the owner or occupant of adjoining property is a private nuisance.
The requirement for malice puts this beyond simple interpretation even though an appellate case already determined that a row of trees can be considered a spite fence. The advice of a licensed California real estate attorney should be sought to understand how this could impact you.
Perhaps the most common dispute around fences is actually defining where the property line is located. If a fence is being built by one property owner without the agreement of the adjacent property owner, it needs to remain off the property line. Of course no property owner wants to give away any of their land because of a fence.
Most often, replacing an existing fence will be agreed to by a simple discussion between neighbors. If that is not the case, a survey may be required. Consult a California licensed real estate attorney.
There are two basic points regarding trimming trees near property lines.
If the trunk of a tree is wholly on the land of one owner, the tree belongs exclusively to him even if the roots grow into the land of another. §833
California Civil Code 3346 considers that the encroachment of branches and roots onto your property to be a nuisance.
If you are wanting to do significant trimming you would be wise to hire the service of a licensed and bonded professional.