There are multiple definitions - click on the translation that matches your context.
accession means:
Substantiv
full
noun
Given the fact that the etymology of this English word is directly related to the Latin cognate that describes the historical legal basis for this German concept and matches its very function, this English word choice is the best one.
a new item added to an existing collection (eg of data, paintings, books etc); an amount added to an existing quantity of something (eg wealth)
The courts have defined income to include all accession to wealth, including money, oil, precious minerals and other assets.
a property owner's right to everything that is added to the property, either naturally or through improvements (eg further building work)
It is through accession that property in minerals belongs to the rightful title holder.
the act of agreeing to something
A settlement was finally reached following Prendi's accession to Romantik's demands.