accessio

Accessio is a Latin term meaning an addition or increase, used in property law to describe the transfer of ownership that occurs when a thing is augmented by another thing. The concept is most closely associated with civil-law traditions and is often translated as accession.

In general, accessio covers two main forms: natural accession and artificial accession. Natural accession refers to the fruits or products that naturally belong to the owner of the principal thing, such as crops produced on land or offspring linked to a parent object. Artificial accession covers additions made by human action, such as buildings, improvements, or attachments that become integrated with a main property. The prevailing principle in many civil-law systems is that the owner of the principal thing accrues ownership of the entire combined object, including the accession, unless there is a contractual arrangement to the contrary.

There are important exceptions and complications. If the accession is detachable and can be separated without damage to either party, rules may permit recovery or compensation rather than outright ownership of the accession. When the principal thing and the accession are owned by different parties by contract, the governing agreement typically determines title. Modern civil-law codes codify these rules and apply them to various contexts, including land with structures, movable property that is permanently altered, and other forms of augmentation.

In practice, accessio provides a framework for resolving disputes about ownership when property is enhanced or altered, coordinating title with the nature of the addition. In common-law jurisdictions, similar questions are addressed through concepts such as fixtures and improvements, but the term accessio is primarily used in civil-law contexts.