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 Oregon, adverse possession laws allow a person to acquire legal ownership of a property if they possess it in a manner that is open, notorious, exclusive, continuous, and hostile for a statutory period. The statutory period in Oregon is 10 years, as per ORS 105.620. To claim adverse possession, the possessor must have maintained a reasonable belief that they were the property owner throughout that period. This belief must be based on an actual mistake, and the possessor must have acted as the typical owner would, maintaining the property and paying taxes on it. If these conditions are met, the possessor may be able to obtain legal title to the property, potentially altering the boundary lines. It's important to note that adverse possession claims can be complex and typically require the involvement of an attorney to navigate the legal process and to potentially litigate the claim in court.