The site of the Houses of Parliament (including Big Ben) is the Palace of Westminster, a royal palace and former residence of kings. The layout of the Palace is intricate, with its existing buildings containing nearly 1,200 rooms, 100 staircases and well over two miles of passages. Among the original historic buildings is Westminster Hall, used nowadays for major public ceremonial events.
Palace of Westminster
London
SW1A
http://www.parliament.uk/directories/hcio/tours.cfm
Tours for overseas visitors are only available during the Summer Opening of Parliament. At present, UK residents can tour during session on Monday Tuesday and Wednesday mornings and on Fridays – all day if the Commons are not sitting, late afternoon if they are. Check before travelling.
UK residents, during session: People should contact their local Member of Parliament or a Peer, if they know one. If you're unsure of your local MP's details, contact the House of Commons Information Office. The MP or Peer concerned can usually arrange for visitors to take a tour in English or another language. Applications should be made as far in advance as possible.