Thank you Chaharper and Octavius for the pics. I have found a little history from a souvenir book.
Until the Coronation of William IV the processions into the Abbey were arranged in Westminster Hall, adjoining the Houses of Parliament. This practice ended with the Coronation of George IV and involved the abolition of a number of ceremonies ..... which used to take place there, and for every Coronation since, beginning with that of William IV, a temporary Annexe has been built outside the west door of the Abbey.
Responses are not allowed!