I don't know about an apartment in town other than his rooms in the Town Palace, but I do know that he lived in D'Argens' house for a while, which doesn't exist anymore but was to the south of this map section, at the Havel bay and next to the place where FW4 later built the Dampfmaschinenhaus to finally power the Sanssouci fountains (that one still exists). (The modern Voltaireweg looks like it might be right in the place where training maneuvers used to be held.)