His architectural projects are of international stature and introduced a neoclassical style, with Italian and English influences, to the region.
He designed a large number of châteaux, abbeys and churches in Belgium, many of which were damaged after the French Revolution.
There he worked with Luigi Vanvitelli and came into contact with Robert Adam, Giovanni Battista Piranesi, Johann Joachim Winckelmann and Charles-Louis Clérisseau.
In 1767 he was appointed court architect to the Governor of the Austrian Netherlands, Charles-Alexandre de Lorraine for whom he reconstructed the Château of Mariemont, which was demolished in the aftermath of the French Revolution.
Dewez's adversaries, envious of his success as court architect, accused him of failures and fraud in the execution of this project.