The costs of the extension to the Spandau city centre rose to unexpected heights: it consumed 680 million German marks as a result of the need to undercut the Havel river and the swampy ground in the surrounding area. Construction began on 4 July 1977. Several variants were investigated. One plan was to lead the subway through the Haselhorster village center, with a downside that the Havel had to be undercut at one of its widest points. Another variant was that the subway would cross the existing Spandau suburban railway of the S-Bahn and the subway would be led from the south to Spandau city hall. This plan crystallized very late, as the plan envisaged to lead the U7 over Falkenseer Platz into Falkenhagener Feld. This resulted in the disadvantage that the Spandau old town could not be connected, which was the main goal of the extension. Finally, they decided to continue the route along the Nonnendammallee or the street Am Juliusturm, south past the Spandau Citadel and then directly to the old town and on to the Spandau town hall, with the development of Falkenhagener Feld taken over by the extension of the small profile route of the line U2 of Ruhleben. On 1 October 1984, the last section of the U7 from Rohrdamm to Rathaus Spandau was opened, with the then Federal Chancellor of Germany, Helmut Kohl, in attendance.