The Cabinet War Rooms began to be develped in the 1930's as a centrally-located base for government during what was then the possibility of a conflict involving aerial bombardment. Its humble location, just underneath the Office of Works, was chosen for the building's strength and convenient location between 10 Downing Street and the Houses of Parliament. It was completely converted from its original purpose, a general storage basement, only a week before the German invasion of Poland and the subsequent declaration of war by Britain.
One enters the Cabinet War Rooms through a modern entrance: the sandbags surrounding the door are fake, placed when the Cabinet War Rooms were opened to the public. The original entrance was through a door in the cabinet offices, underneath which the Cabinet War Rooms are located.