The original metro plans had not called for the Østensjø Line to run further than to Bøler. As housing development was gradually forcing its way further south in Nordstrand, the municipal council saw the need for extensions to the line. The section from Bøler to Borgerud was approved on 8 March 1963 and construction started immediately. The section from Borgerud to Skullerud was approved of on 31 January 1964. The total costs of the line from Helsfyr to Skullerud ended at 39 million Norwegian krone.