The Household Cavalry is to open Horse Guards to the public for the first time ever as part of this year's London Open House.
The 1745 building in Whitehall is a constant draw for tourists, who flock to the courtyard and parade ground but never go through the doors to William Kent's building, influenced by Inigo Jones.
Other firsts for the event on 18 and 19 September include the inside of the bbc's 1925 Bush House by Helmle & Corbett; Australia House, also in Aldwych, designed in 1935 by Marshall Mackenzie; and Charles Holden's 55 Broadway, a 1929 hq for London Transport.
The Guardian will publish the full Open House directory in its 'Guide' on Saturday 7 August. London Open House is running a helpline from mid June, 0891 600061. Its web site is www.londonopenhouse.demon.co.uk