Seaford House, originally called Sefton House, is one of the grandest surviving aristocratic mansions in London, England. It is the largest of the three detached houses which occupy three corners of Belgrave Square in the exclusive district of Belgravia. Seaford House is a white stucco building with four main stories. Dated 1842, it was designed by one of the leading architects of the 19th century, Philip Hardwick for the Earl of Sefton. It is a Grade II* listed building.
In 1902, Sefton House was remodeled for Lord Howard de Walden, who was also Baron Seaford. It was at this time that it became known as Seaford House. Howard de Walden had a unique marble staircase, friezes and paneling installed. It is now the home of Royal College of Defence Studies, and is usually open to the public free of charge on Open House Weekend each September.