Adverse possession—sometimes called squatter's rights—occurs when a person or entity who does not own a certain piece of real property occupies or uses it in a way that is adverse to the owner of the property—meaning the use is in conflict or at odds with the property owner’s interests.

If this adverse possession or use of the property meets the legal standard for adverse possession—often described as use that is continuous, exclusive, hostile, open, and notorious—and continues for the prescribed period of time (usually specified in the state statute)—the person or entity adversely possessing or using the property will acquire ownership of the property (title). This ownership will result in the changing of boundary lines.

Because the required adverse possession must be open, obvious, and adverse to the owner’s interests, title may only be acquired by adverse possession to the portion of property that is used in that way.

Adverse possession laws vary from state to state and may be located in a state’s court opinions (also known as common law or case law) or, more often, in its statutes.

In California, adverse possession laws allow a person to obtain legal ownership of a property by occupying it for a certain period of time, provided specific conditions are met. The requirements for adverse possession in California are governed by state statutes, particularly the California Code of Civil Procedure Sections 315-330. To claim adverse possession, the occupant must demonstrate possession that is hostile (without the owner's permission), actual (exercising control over the property), open and notorious (visible and obvious to anyone), exclusive (not shared with others), and continuous for a period of five years. Additionally, the claimant must have paid the property taxes during that period. If all these conditions are met, the individual may be able to acquire legal title to the property, potentially altering the boundary lines. It is important to note that adverse possession is a complex area of law, and those considering or contesting a claim should consult with an attorney to understand the specific implications and processes involved.