By 1866 a postal staff of 225 was handling around 400,000 letters a week. Initially
there was sufficient space to allow part of the building to be allocated to the Board of Lunacy and
Ordnance Survey. However, it was not long before these departments were asked to move out as the
rapidly increasing postal business exhausted all the accommodation. This was largely due to the
electric telegraph being taken over by the Post Office in 1870.
As business continued to grow, a little over 20 years after the opening of the GPO it was clear that
major extensions were needed to the building and W.W.Robertson, a Board of Works architect, was
commissioned to increase the accommodation at a cost of £30,000.
