Time Out says
This Grade I listed building in the Palladian style at the heart of Whitehall was the headquarters of the British Army for a hundred years. The current building, replaced an earlier one on the site of the former tiltyard of Westminster Palace, and was built between 1751 and 1753 by John Vardy to a design by William Kent. Inside, the Duke of Wellington's office remains as it was c1842. Located within the building is the Household Cavalry Museum, where as well as the historical displays you can look through a glazed partition to see the working horses in their eighteenth-century stables. Outside is Horse Guards Parade, the setting for many of London's most spectacular displays of pageantry, including the regular Changing of the Guard, the grand military ceremony of Beating Retreat and Trooping the Colour, which takes place in June each year to commemorate the sovereign's official birthday.
During Open House London 2014, Horse Guards will be open to the public on Saturday September 20 and Sunday September 21, 10am-3.30pm. Tours lasting 45 minutes will run every half an hour but expect queues of up to two hours. Entry is via Horse Guards Arch on the south side.