The planned extension, which would connect to line 7 (now 7bis) at the station Pré-Saint-Gervais, stretched . It was constructed under the avenue Gambetta to Porte Lilas, where a turnaround loop was installed under the city walls. The terrain of the area, which was especially moist and unstable—due to the ancient Saint-Fargeau Lake—made construction difficult. As a result, the tunnel was built in a layer of gypsum, very deep underground; stations were contained in a reinforced vault, with elevators linking the platforms to the surface. Beyond the Porte des Lilas, a double-track tunnel extended by two single-track tunnels—the "voie des Fêtes" (south tunnel) and the "voie navette" ("shuttle line", north tunnel) connected the extension to line 7—allowing the latter to be extended to Porte des Lilas in the future.