The line has received one extension after the metro conversion. Additional housing in the Mortensrud area resulted in plans to a metro connection to the area. Also the Lambertseter Line was considered to be extended to the area, but the Østensjø Line was eventually landed upon. Initial plans called for construction to commence in 1992 and be completed in 1994. However, construction did not start until 1995. The extension is built to allow speeds of , and represents a considerable longer distance between stations than what is normal on the network. Most of the section is in tunnels, though there are also two viaducts. The extension, excluding the new station, cost 215 million krone. The station cost 35 million krone. It was taken into use on 24 November 1997, but not officially opened until 4 January 1998.