The contract includes a complete rebuild of the existing District and Circle Line station and lowering of the track level to enhance the facility and to provide a base slab for the New Parliamentary Building which has been constructed on top. The new Jubilee Line Extension station was originally to be sited beneath Parliament Square but after objections from the House of Commons New Buildings Works Committee London Underground decided in December 1990 to build the new station directly below the existing one. This tight location, in particular its proximity to the Big Ben clock tower meant that the new Tube lines had to be built one on top of the other rather than side by side. Certainly the resulting 40m station was the most technically challenging on the entire project. The new station is expected to be used by 15,400 passengers in the peak morning rush hour.