When the system was first designed, it was planned that further extensions would be built at a later date. The proposal was that after a few years of operation of the first section, testing out the various systems and stock used, that 3 extensions would be built.
These comprised a direct line from King Edward Building to West Central District Office, continuing to the South Western District Office at Howick Place, near Victoria. A loop line from Mount Pleasant to Euston, the North Western District Office in Eversholt Street, St. Pancras, Kings Cross, and back to Mount Pleasant; one from Mount Pleasant to the Northern District Office in Upper Street; lastly one from King Edward Building to Cannon Street, London Bridge, the South Eastern District Office in Borough High Street, the South Eastern Parcels Office, Waterloo, and the South Western District Office.
Although nothing ever came of these proposed extensions, they were kept in mind for a surprisingly long time. Short sections of the extension were started whilst the main section was being built, most notably at Mount Pleasant, but only extended a few yards.
Even in recent times, proposals were made to extend the line, some four and a half miles, to the new central sorting hub at Willesden. Needless to say this was abandoned due to the estimated cost of the extension. At the time, it cost over a million pounds per mile to excavate a deep level tube tunnel.