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 Washington State, adverse possession is a legal doctrine that allows a person to claim ownership of land that they have been occupying under certain conditions. To claim adverse possession in Washington, the occupant must use the property in a manner that is open and notorious, exclusive, continuous, hostile (against the interest of the true owner), and for a statutory period, which is typically 10 years under RCW 4.16.020. The use must be uninterrupted and the claimant must treat the property as their own, without permission from the true owner. If all legal requirements are met, the person claiming adverse possession can file a lawsuit to obtain legal title to the property. It's important to note that the specifics of each case can affect the outcome, and an attorney can provide guidance on the likelihood of a successful adverse possession claim.