At the beginning of the 20th century, 17 Belgrave Square was lived in by Pandeli Ralli, who, like the Royal College of Psychiatrists 100 years later, was already a long-standing tenant. At this time most of the houses in the Square were occupied by private tenants and their domestic staff, with the coachmen, grooms and their families living in the mews houses at the rear. As the 20th century went on this changed, and by the 1990s most of the tenants were companies or organisations. The Voters' list shows that there are very few private individuals living here and the names of occupiers displayed at the entrances to buildings indicate that some have three or more organisations as tenants. Number 17 is one of the few that has a single occupier.

Pandeli Ralli, who was born in Marseilles, was a British subject whose family had come to England from the Greek island of Scio in the early 1800s. He became an MP and it is said that his house was used by Lord Kitchener as a social headquarters during the First World War. His family, who were very wealthy, helped finance the Greek struggle for independence from Turkey. He died in 1928, having been a tenant of Number 17 for about 60 years.

The next tenant was Leontine, Lady Sassoon. She too had overseas connections, for her husband's family, the Sassoons, came originally from Baghdad. She lived here from 1929 until 1942 and, like Pandeli Ralli and Lord Kitchener during the previous war, kept an open house for the troops during the Second World War. During that war, part of the property was used as a supply depot for the Red Cross. Lady Leontine left in 1942 but retained the tenancy until she died, aged over 90, in 1955.

Examples can be found in 20th century novels of Belgrave Square as an area where the wealthy lived. A couple described by Nancy Mitford in Highland Fling (Reference Mitford1931) were “in many ways extremely economical. Unlike the type of young married couple who think it essential to have a house in the vicinity of Belgrave Square and a footman, they preferred to live in a tiny flat with no servants except an old woman and a boy”. Nicholas, the narrator in Anthony Powell's A Dance to the Music of Time (Reference Powell1962) finds that one evening he is asked to two dances “And both of them in Belgrave Square” where the one he choses to go to is “densely packed with girls and young men… even on the way up the stairs”. And the poverty stricken heroine in Charlotte Bingham's (Reference Bingham1983) Belgravia flees from her very rich would-be seducer's “house in Belgrave Square back to Mary's, with the knowledge that it was at such times, when life was at its most real, that she most disliked it”.

Finally, not in fiction but in reality, Christobel Bielenberg in The Past is Myself (Reference Bielenberg1968) records an evening in wartime Germany where she meets an old acquaintance “in the best of form, as British as the flag… We might have been back in Belgrave Square”.

It is clear from Kelly's Directories of London that until the 1950s not only were most of the houses occupied by families but that many of these families were members of the aristocracy or gentry. In the 1909 Directory, for example, nearly half the occupiers listed have titles. Although some of the houses were put to other uses in the First World War, for example Number 13 was used by the St John Ambulance and Numbers 19 and 43 were annexes to King Edward VII's Hospital for Officers, it was not until after the Second World War that the modern pattern of occupation by embassies and organisations developed.

Number 17 was taken over by the Institute of Metals in 1956 and the College came to Number 17 in 1974. Thus in the 160 years since it was first occupied in 1839 17 Belgrave Square has only had five different tenants.