Although untrained, his skills and knowledge, gained through experience, made him one of the most sought after architects of his day.

He played a major role in the development of towns in the area such as Pápa, Eger and Veszprém.

Nevertheless, he was unable to finish the work and it was left up to Jozsef Grossmann to complete after Fellner's death.

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.

Fellner completed the Liceum building in Eger which was initially planned by Jozsef Gerl of Vienna.