(civil law) the right to all of that which your property produces whether by growth or improvement WordNet 3.0 & Open English WordNet

Accession has different definitions depending upon its application. Wikipedia

A mode of acquiring property, by which the owner of a corporeal substance which receives an addition by growth, or by labor, has a right to the part or thing added, or the improvement (provided the thing is not changed into a different species). Wiktionary