Time Out says
Completed in 1711 this Grade I listed mansion was designed by Christopher Wren, his son of the same name and Sir James Pennethorne to the specifications of Sarah Churchill, Duchess of Marlborough who wanted her new house to be 'strong, plain and convenient and good'. It was the home of the Dukes of Marlborough for over a centaury and later of Edward VII and Queen Mary. It is now the HQ of the Commonwealth Secretariat and the Commonwealth Foundation.
The ground floor of the building will be open to the public for Open House London 2014 on Saturday September 20, 10am-3.30pm. 45 minute tours will every half an hour but get there early as there can be queues up to two hours long at peak times.