In 1764 Bishop Carl Eszterhazy put Fellner in charge of building works and planning in Eger, and this was to become one of the most important periods of his career. His first work for the city was the stairway section of the Bishop's palace and also its chapel in 1766. He also planned the bishop's palace in Veszprém. Fellner completed the Liceum building in Eger which was initially planned by Jozsef Gerl of Vienna. Fellner's last great work was the Parish Church at Pápa in 1773.
