Inside Britain's most expensive home - In and Out Club

The entrance hall alone is bigger than many London flats and an 85ft-long ballroom will put many five-star hotels to shame.

These are the first images of the interiors proposed for what will be Britain’s most expensive residence, with a price tag of about £200 million.

The Grade I-listed property in Piccadilly, known as Cambridge House, has been left forlorn and decaying for more than 15 years.

Now the billionaire Reuben brothers, who paid £150 million for this and the surrounding buildings last summer, have submitted planning applications for a “sensitive conversion”.

Under the plans, the house at 94 Piccadilly, also known as the In and Out Club, will provide 53,426 sq ft of space. This works out at about £3,750 per square foot, a record for Mayfair. For that, the buyer will get 48 rooms, of which 11 will be bedrooms, with the master bedroom suite occupying most of the 7,846 sq ft first floor. The central stairhall leading up to the ballroom is 55ft high under its ornate cupola.

There will be nine bathrooms, two guest powder rooms and eight cloakrooms. A basement wine cellar will be able to hold 35,000 bottles and there will be two lifts plus a car lift to the 12-vehicle car park. The courtyard will be excavated to form a 40ft underground pool with spa, gym and beauty salon.

Originally built for Charles Wyndham, 2nd Earl of Egremont, by architect Matthew Brettingham in 1756-1761 in the Palladian style, from 1829 the house was the London residence of Prince Adolphus, Duke of Cambridge, when it became known as Cambridge House.

After the duke died in 1850, the house was purchased by Lord Palmerston — who served twice as prime minister — and on his death it was bought by the Naval and Military Club. In the late 1860s, the club became known as the “In and Out”, from the signs on the entrance and exit gates.

The Naval and Military was there until 1996 when its lease expired and the club moved to 4 St James’s Square, while Cambridge House was sold to businessman Simon Halabi for £50 million. In spite of his plans to convert it into a members club and hotel nothing happened until 2009, when Halabi’s companies went into bankruptcy.

A spokesman for the Reuben brothers said: “A key aim of the proposal is to remove buildings from the Heritage at Risk Register and substantially enhance this part of the Mayfair conservation area. The properties have been neglected for many years, resulting in extensive deterioration.”