Today, Westminster Palace is the seat of the British government. “The Palace of Westminster is the meeting place of the House of Commons and the House of Lords, the two houses of the Parliament of the United Kingdom. Commonly known as the Houses of Parliament after its tenants, the Palace lies on the Middlesex bank of the River Thames in the City of Westminster, in central London. Its name, which derives from the neighbouring Westminster Abbey, may refer to either of two structures: the Old Palace, a medieval building complex that was destroyed by fire in 1834, and its replacement New Palace that stands today. For ceremonial purposes, the palace retains its original style and status as a royal residence.” http://en.wikipedia.org/wiki/Palace_of_Westminster
No floor plans of what Westminster Palace looked like in the middle ages still exist, but we do know a few things:
“When William the Conqueror’s son, William Rufus, came to the throne in 1087 he decided to build a new Palace which would rival the size and splendour of Westminster Abbey. This Palace was never fully completed, but its major Hall still exists to this day and is now called Westminster Hall. It is this chamber which has since become the site of some of the most famous Trials in English History.
In the Twelfth Century the White Hall was built to the South of the Great Hall, a statue of Richard I now stands on its site. At the same period, another chamber was added to the North East corner of Westminster Hall.
During the reign of King Henry III in the Thirteenth Century, the Painted Chamber, the Queen’s Chamber and the Queen’s Chapel were added to the South East of the site, with access to the Garden and the River. It was probably within the Painted Chamber that the Death Warrant of Charles I was signed in 1649. By the end of the thirteenth century Westminster Hall had come to be the site of the Courts of Kings Bench, Chancery, and Common Pleas. All lay within the perimeter of the Hall, except the Court of Exchequer which was in a building adjoining the North of the West wall.” http://www.explore-parliament.net/nssMovies/11/1111/1111_.htm
One of the oldest buildings of Westminster is still standing–the hall, which was built in 1097 “under William II (Rufus), the son of William the Conqueror, and was completed two years later. He had conceived the project to impress his new subjects with his power and the majesty of his authority.
According to one story, when the King first inspected the Hall, one of his attendants remarked that it was much larger than needed. The King replied that the Hall was not half large enough, and that it was a mere bedchamber when compared to what he had in mind.
But the Hall was indeed by far the largest hall in England, and probably in Europe at that time. Measuring 73 by 20 metres (240 by 67 feet), it had a floor area covering 1,547 square metres (about 17,000 square feet), with a length of almost four cricket pitches end-to-end.
Indeed, the Hall was so large that other halls were needed at Westminster for normal use, and the royal household usually ate in a smaller hall nearby.
The great mystery about the Hall is the form of its original roof. Not until the 13th or 14th century could carpenters create roofs significantly wider than the length of the available timber, and so it was assumed that a single or double row of columns was needed to support the Hall’s roof.
However, recent archaeological explorations found no evidence of these, and that the roof may have been self-supporting from the beginning.
The Hall was enclosed with stone walls fully two metres, or six feet thick; these largely remain today, though heightened and refaced.
Inside the Hall was an arcade with large arches and windows and a wall passage around all four sides. Above the windows was a chequer-work pattern of light and dark stones.
The inside walls were plastered and painted, and decorative hangings were draped from the arcade.” http://www.parliament.uk/about/living-heritage/building/palace/westminsterhall/architecture/early-history/
This is a floor plan of it now: