If a person occupies someone else's land without legal objection from that person for a specified period of time, and does so openly and under a claim of exclusive possession, that person can end up owning the land in spite of later protests. This law has a long history and was enacted to prevent absentee landowners from neglecting property they owned.
In California, Section 325 of the Code of Civil Procedure states the criteria for a claim of adverse possession.
(a) For the purpose of constituting an adverse possession by a person claiming title, not founded upon a written instrument, judgment, or decree, land is deemed to have been possessed and occupied in the following cases only:
(1) Where it has been protected by a substantial enclosure.
(2) Where it has been usually cultivated or improved.
(b) In no case shall adverse possession be considered established under the provision of any section of this code, unless it shall be shown that the land has been occupied and claimed for the period of five years continuously, and the party or persons, their predecessors and grantors, have timely paid all state, county, or municipal taxes that have been levied and assessed upon the land for the period of five years during which the land has been occupied and claimed. Payment of those taxes by the party or persons, their predecessors and grantors shall be established by certified records of the county tax collector.
If you own property you should be aware of the possibility of adverse possession. Seek the advice of a local attorney familiar with real estate law if your property is at risk.
Do Fences Establish Boundary Lines?
From time to time neighbors discover that a fence between their properties is not located on the true property line. Unfortunately your attorney may not be able to tell you with certainty what the effect of the fence will be. Almost never will the fence determine property ownership but limited rights could be given to the encroaching property owner.
In the 1996 case of Silacci vs. Abramson the trial court determined that Abramson had acquired "an exclusive prescriptive easement for the fenced-in portion of Silacci's property. The Court of Appeal reversed the judgment saying that an exclusive prescriptive easement amounted to ownership.
In the June of 1996 case of Mehdizadeh vs. Mincer the trial court entered a judgment that the fence because of the "doctrine of agreed boundary" did give the encroacher rights to the property. The Court of Appeal reversed the decision. The Court of Appeal held that a long period of acquiescence without evidence as to uncertainty was insufficient to establish an agreed boundary. The court said "Where the evidence does not satisfy the requirements of the doctrine, the law should not employ the agreed-boundary doctrine to 'trump the boundary established by the legal records' ..."
From 1996 until 2001 it was generally accepted that a misplaced fence does not create a prescriptive easement because a fence defines an exclusive use to the exclusion of the true owner.
In August 2001 the Second District Court of Appeal (Los Angeles) applied the "relative hardship doctrine" to award rights to the encroacher. The court used "equitable protective interest" terminology rather than easement.
So although ownership has not been successfully challenged (property taxes have not been paid by the encroacher), the use of encroached property is difficult to predict. Please see a detailed discussion from the Law Office of Peter N. Brewer at: http://www.brewerfirm.com/articles/article-legalsausage.html