This chapter examines Leon Jaussely’s 1919 Projet lauréat de la section générale du concours du plan d’extension de Paris (Plan d’extension) as a result of the 1919 loi Cornudet, which proposed that any French city with a population greater than 10,000 submit a ‘projet d’aménagement, d’embellissement et d’extension.’ The legislation grew in part from the influence of the Musée social movement, which became the focal point of studies of hygiene, social reform, and ultimately urbanism. The Musée social pushed for the better regulation of growth and the incorporation of more green space into French urban agglomerations. Jaussely joined with Roger-Henri Expert and Henri Sellier to submit their Projet lauréat, which won the première prime prize, Jaussely used the map to address what he, Expert, and Sellier wished to see in a modern French metropolis. Jaussely’s sizeable hand-painted Plan d’extension marks the beginning of modern urban planning in France. Jaussely incorporates recommendations healthy living environments for residents and visitors. New parks and cités jardins in the suburbs incorporate usable green space. In addition, new ports, aérogares, and rail stations on the agglomeration’s edge ensure ease of movement over the large expanse of territory.