definition of accession:
a right of an owner to anything which may be added to his or her property by another party. While the right of accession may be applied to issues of personal property, it is most frequently an issue in real property.
accession, in an example:
Fiona and Fergus own a rental property which they leased to their tenant, Roger. During his occupancy and without permission Roger built a small carport. That carport, by accession, belongs to the owners of the property, Fergus and Fiona.