The last great aristocratic mansion in Park Lane is for sale with a price tag of £40m. The most expensive house on the London market this year, 100 Park Lane — top prize on the Monopoly board — could also be the capital’s costliest restoration project.

The 18th-century house overlooking Hyde Park, now used as offices, has residential planning permission to be converted back into a family home. The buyer will have to spend at least £10m to restore the house to its former glory.

Plans have been drawn up by Halpern Architects to return the Mayfair property to its Regency splendour, with 14 reception rooms and 17 bedrooms, a double-height ballroom, 81ft picture gallery and a conservatory. There is also potential for four staff flats, a swimming pool, gym, two lifts, parking for 26 cars and a courtyard garden.

The 125-year lease on 100 Park Lane, otherwise known as Dudley House, is for sale through Savills on behalf of Hammerson, the property group that built the Bullring in Birmingham and Brent Cross shopping centre. The freehold is owned by Grosvenor Ltd, the Duke of Westminster’s property company.

Savills has conducted 20 viewings of the property, in reach of “only the top end of the billionaire market”. Middle Eastern royals and Russian oligarchs are the target audience. So far, six parties have apparently expressed serious interest.

“There are a handful of big houses in Mayfair but they rarely come up for sale,” says Jonathan Hewlett of Savills. “Buyers love the address and now they want to own it,” The new owner will also have to furnish the 39,000sq ft house — about 65 times larger than the average London flat. They might scheme their decor around two portraits by Sir Joshua Reynolds of the first owner, Viscount Dudley, and his wife which could be included in the sale.

The house traces its origins to 1736, but it was 1756 before the plot of land was leased to the first Viscount Dudley, whose family retained possession of the property until 1940. Substantial alterations were carried out by the fourth viscount, who had the front of the house partly stuccoed and the staircase lined with mirrors in imitation of Versailles.

The mansion was later extended to include a picture gallery and a giant ballroom. Here Sir John Ward, a member of the Dudley family, and his wife Jean Reid, daughter of the American ambassador, presided over some of the most glittering occasions in the pre-war social calendar.

The kitchens were among the largest in London — one for the food prepared for the family, another for the servants who served them, and another for the servants who served the servants.

In 1938, Sir John died and the house was later requisitioned as offices by the government. It was bombed during the Blitz, and the severe damage opened the way for it to be converted to offices. Hammerson bought the long lease in 1969 and upgraded the interior. Sir Basil Spence, who designed Coventry Cathedral, oversaw the restoration, constructing a marble-floored atrium. Rumour has it that when they were regilding, they had to inform the Treasury due to the amount of gold being used.

So far this year, 13 houses have sold in London for more than £10m. Witanhurst in Highgate, London’s largest residence after Buckingham Palace, is on sale for £32m.

Last year, 22 houses in the capital sold for more than £10m. Lakshmi Mittal, 55, the third-richest man in the world, paid £70m for a 12-bed home in Kensington Palace Gardens and Leonard Blavatnik, a Russian oil tycoon, snapped up a house on the same road for £41m.

Hewlett believes that once restored, 100 Park Lane could sell on for £70m.

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