A more thorough explanation:

Definition: Droit d'accession (drwah dak-ses-syawn), n. [French “right of accession”] Civil law. The right of the owner of a thing to whatever is produced by it or is united with it, either naturally or artificially. This means that if you own something, you also own anything that is produced by it or added to it. For example, if you own a tree, you also own the fruit that grows on it.

The equivalent of the Roman specificatio, the right includes, for example, the right of a landowner to new land deposited on a riverbank and the right of an orchard owner to the fruit of the trees in the orchard. This means that if you own land next to a river and the river deposits new land on your property, you own that new land. Similarly, if you own an orchard, you own the fruit that grows on the trees in the orchard.

The civil law rule is that if the thing can be reduced to the former matter it belongs to the owner of the matter, e.g. a statue made of gold; but if it cannot so be reduced it belongs to the person who made it, e.g. a statue made of marble. This means that if something can be turned back into its original form, the owner of the original material owns it. For example, if a statue is made of gold, the owner of the gold owns the statue. But if the statue is made of marble, the person who made the statue owns it.