Cambridge House is a grade I listed mansion on the northern side of Piccadilly (Number 94) in central London, England, named after its most notable owner, the Duke of Cambridge, 7th son of George III. It has also been known as Egremont House, Cholmondeley House, The Naval and Military Club, and the In and Out Club.
The house was built for Charles Wyndham, 2nd Earl of Egremont, by architect Matthew Brettingham in 1756-1761. It was initially known as Egremont House. The house is in a late Palladian style. 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 (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, and became known as Cambridge House. Due to his royal status, that name has persisted.
After the Duke died in 1850, the house was purchased by Lord Palmerston, who was Prime Minister of the United Kingdom for most of the decade from 1855 to 1865. It was his London residence and the site of many splendid social and political gatherings. After Palmerston's death at Brocket Hall in Hertfordshire in 1865, his body was taken to Cambridge House from which his funeral procession departed to Westminster Abbey. 
Shortly after Palmerston's death, Cambridge House was purchased by the Naval and Military Club, which had outgrown its previous premises. The Club came to be known as the "In and Out", from the prominent signs on the entrance and exit gates of 94 Piccadilly.
In 1999, the Club moved to new premises, having sold Cambridge House in 1996 to 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 (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 won 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 valued in the region of £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.
- Sunday Times. Rich List 2004
- "94 Piccadilly, Westminster, City of, Greater London".
- "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.
- Pearce, David (1986). London's Mansions. ISBN 0-7134-8702-X
- Reuben Brothers - real estate holdings
- Reuben Brothers - timeline of purchases and developments
- Country Life Picture Library - interior photos