Cambridge House is a grade I listed townhouse in central London. It sits on the northern side of Piccadilly in the fashionable district of Mayfair. The current name of the house comes from one of its owners, Prince Adolphus, Duke of Cambridge (1774–1850), the 7th son of King George III, but it was originally known as Egremont House, and later as Cholmondeley House. From about 1865 to 1999 it was the home of the Naval and Military Club, and is known colloquially as the In and Out Club due to its prominently signposted one-way carriage drive.
The house, situated in the fashionable parish of St George's, Hanover Square, Westminster, was built in 1756–1761 by Charles Wyndham, 2nd Earl of Egremont (1710–1763), of Orchard Wyndham in Somerset and of Petworth House in Sussex, Secretary of State for the Southern Department from 1761 to 1763, and was thus first known as Egremont House. The house is in the late Palladian style, to the design of the architect Matthew Brettingham. It has three main storeys plus basement and attics and is seven bays wide. As is usual in a London mansion of the period, the first floor (piano nobile, "second floor" in American English) is the principal floor, containing a circuit of reception rooms. This floor has the highest ceilings and its status is emphasised externally by a Venetian window in the centre.
The house changed hands several times. For several years in the 1820s it was occupied by George Cholmondeley, 1st Marquess of Cholmondeley and known as Cholmondeley House. From 1829 to 1850 it was the London residence of Prince Adolphus, Duke of Cambridge (1774–1850), and became known as Cambridge House. Due to his royal status, that name has persisted.
After the Duke's death in 1850, the house was purchased by Henry Temple, 3rd Viscount Palmerston, who was Prime Minister of the United Kingdom for most of the decade 1855 to 1865. It was his London townhouse and the site of many splendid social and political gatherings. After Palmerston's death in 1865 at Brocket Hall in Hertfordshire, his body was taken to Cambridge House whence his funeral procession departed to Westminster Abbey. 
Later that year, Cambridge House was sold to the Naval and Military Club, which had outgrown its previous headquarters. The Club came to be known as the "In and Out", from the prominent traffic-directing signs on its entrance and exit gates. Members included TE Lawrence and Ian Fleming.
In 1999 the Club moved to new premises, having sold Cambridge House in 1996 to the entrepreneur Simon Halabi for £50 million. Halabi planned to convert the property into a private members club and hotel, part of his Mentmore Towers project, and to build a swimming pool and squash courts underneath the forecourt of the house. However, the building remained vacant after 1999, and fell into a state of disrepair.  Plaster was falling off the ceiling in the first floor rooms, and many floorboards had been pulled up.
In 2009 Halabi's companies went into bankruptcy. In June 2010 Cambridge House and its adjoining buildings, 90–93 Piccadilly (and 42 Half Moon Street), 95 Piccadilly (the former American Club) and 12 White Horse Street (the rear section being vacant land), as well as 96–100 Piccadilly (on the other side of White Horse Street) were all offered for sale through property brokers Jones Lang Lasalle, collectively referred to as the Piccadilly Estate, for in excess of £150m. In June 2011 the site was acquired by the Reuben brothers for a reported £130m through their investment company, Aldersgate. In October 2012 applications were submitted for a full refurbishment into private homes (Numbers 94 and 95) and residential apartments (Numbers 90–93 and 42). 
In April 2013 David and Simon Reuben received approval to develop the property into a 60,600 square foot single home. It is likely to become the UK's most expensive home, estimated to be worth about £250 million after renovation. According to Bloomberg News, "the planning application for Number 94 was approved after the two investors offered to contribute £3.85 million to the construction of affordable housing in the borough."
- Edward Walford (1878). "Mansions in Piccadilly". Old and New London: Volume 4. Institute of Historical Research.
- "Article from (London) Sunday Times, accessed 20 Mar 2016".
- Sunday Times. Rich List 2004
- "94 Piccadilly, Westminster, City of, Greater London". Archived from the original on 2012-12-23.
- "Reuben brothers splash out on Piccadilly properties". Sunday Times. 30 June 2011.
- "Inside Britain's most expensive home – In and Out Club". Evening Standard. 29 October 2012.
|Wikimedia Commons has media related to Cambridge House.|
- Reuben Brothers – real estate holdings
- Reuben Brothers – timeline of purchases and developments
- Country Life Picture Library – interior photos