Complex sentence: In the process of his work in Franche-Comté, Ledoux had become an architect for the "ferme générale", for whom he built a salt storehouse in Compiègne and undertook to plan their vast headquarters on the Rue du Bouloi in Paris.
Split into simpler sentences: In the process of his work in Franche-Comté, Ledoux had become an architect for the "ferme générale". For this company, he built a salt storehouse in Compiègne and undertook the planning of their vast seat on the Rue du Bouloi in Paris.