The Prussian Palaces and Gardens Foundation Berlin-Brandenburg (German: Stiftung Preußische Schlösser und Gärten Berlin-Brandenburg) or SPSG, was founded by a treaty of 23 August 1994 between the German federal states of Berlin and Brandenburg as a public foundation following German reunification. The treaty came into force on 1 January 1995. The foundation is separate from the considerably larger Prussian Cultural Heritage Foundation (Stiftung Preußischer Kulturbesitz).

Source Wikipedia

Address(es)
Postfach 60 14 62,
14414 Potsdam
Germany

https://www.spsg.de/en/home/