[1] In English common law, the added value belongs to the original property's owner.
For example, if a finder discovers a gemstone and in good faith believes it to be abandoned and then cuts it and integrates it into a work of art, the true owner may be limited to recovery of damages for the value of the gemstone but not of the final art piece by way of replevin.
[2] Accession may take place either in a natural way, such as the growth of fruit or the pregnancy of animals, or in an artificial way.
In A Text-Book of Roman Law from Augustus to Justinian, W. W. Buckland discusses a third situation where X builds on Y's land using Z's materials.
[3] X's plants (implantatio) and seeds (satio) acceded irreversibly to Y's soil once they have taken root, but Y must pay expenses if X is in legal possession, since X will have the exceptio dolus malus against Y's rei vindicatio.
Possible tests that could be adopted in deciding this question include: In Roman law, there was no consistency.