Seaford House, originally called Sefton House, is a former aristocratic mansion at 37 Belgrave Square, London, England. It is the largest of three detached houses which occupy three corners of Belgrave Square in the district of Belgravia. Seaford House is a magnolia stucco building with four main storeys. The main vestibule of Seaford House was used as Titanic's Grand Staircase in the 1979 miniseries SOS Titanic. Seaford House later stood in as the exterior of the home of Maggie Gyllenhaal's character Nessa Stein in the BBC and SundanceTV television miniseries The Honourable Woman in 2014.