The official website for The Settlers of Catan

Question about Base Game Settlements and Cities - Over the course of the game, may I build further settlements and roads somewhere on the game board that don’t connect to my already existing settlements and roads?

Answer

No. This is only possible during the set-up phase. Afterwards, each settlement must connect to at least one of your own roads, and each road must connect to one of your own settlements/cities or roads.