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 Illinois, adverse possession laws require that a person must occupy a piece of property in a manner that is hostile, actual, open, exclusive, and continuous for a statutory period of 20 years (735 ILCS 5/13-101). The occupation must be such that it would give the legal owner a cause of action for ejectment. The adverse possessor must not have the owner's permission to be on the property, and their possession must be obvious to anyone, including the owner. If all legal conditions are met over the uninterrupted period, the adverse possessor can potentially acquire legal title to the property. However, acquiring title through adverse possession is complex and typically requires a court action to clear the title and formally transfer ownership. It is recommended that anyone considering or contesting a claim of adverse possession consult with an attorney to understand the specific requirements and legal implications.