It is on one of London’s most prestigious squares, with more than 21,000 sq ft of living space, 20ft ceilings and luxury fittings throughout. But will anyone actually pay £100m for No 10 Belgrave Square?

That is the question being asked by the handful of estate agents who make their living selling the capital’s most expensive homes. As the builders put the finishing touches to the inside of the six-floor, white-stucco-fronted building, Savills, the estate agent, is looking for someone with both the resources and inclination to spend so much on a home.

The property, which has a huge basement and a mews house behind, has been gutted and revamped by Musa Salem, its Lebanese owner, a private developer. Having bought the building’s freehold, he has aimed, according to one agent familiar with the property, to recreate the ambience of his own, slightly more modest, home in “billionnaires’ row” – The Bishops Avenue in north London.

The sale is shrouded in secrecy that is extreme even for this level of the market. Buying agents, normally involved in such deals, are not being allowed in to look “on spec”, and any potential clients they produce will first be vetted.

If Salem were to get his price, it would break the record for this type of property: £80m paid for a house in Upper Phillimore Gardens, in Kensington. It was bought by Elena Franchuk, a Ukrainian philanthropist, when the very top end of the London market was near its peak in February last year.

Belgrave Square is also home to Oleg Deripaska, the Russian metals magnate, Sheikh Mohammed, the ruler of Dubai, as well as a handful of embassies. Can’t quite manage £100m? On the opposite side of the square, No 31 has also come onto the market at a marginally more affordable £80m.

The eight-bedroom, 20,000 sq ft house is being sold by the Saudi Juffali family, following the death of its owner, who bought it for £33m in 2006. Savills’ rival, Knight Frank, is handling the sale. So why the near-threefold price increase?

“It’s in really wonderful condition,” says one agent who has been inside. “The design shows Middle Eastern influences and has plenty of marble and decorative features, but all done with taste and to a standard you seldom see.” It has a gym, a pool, a mews house and a garage room large enough to house any prospective buyer’s collection of Bentleys.

Laid out by Thomas Cubitt for the 2nd Earl Grosvenor, later the 1st Marquess of Westminster, in the 1820s, the square, which forms the heart of Belgravia, is one of the grandest in London. The original scheme consisted of four terraces – three of 11 grand houses and one of 12 – with detached mansions in three of the corners and a private garden in the middle.

Yet with even the super-rich touched by the global economic collapse, both houses could struggle to sell – at least at these prices. “At the ‘uber-prime’ end of the market, with few exceptions, buyers expect a price adjustment. To ask £100m for such a house and expect it to be paid is ridiculous,” says top-end property agent Charles McDowell.

“The lower end, below £5m, is experiencing considerable activity, mostly made up of European cash buyers,” he says. “The middle-price sector, at £5m-£10m, is relatively sluggish, with the main problem being lack of supply. The upper end of the market, above £10m, is again patchy. There are buyers out there, but they remain cautious, as they aren’t finding enough choice.”

Andrew Langton, chairman of agent Aylesford, which has been involved in some of London’s biggest property deals, agrees. “Unlike in the previous property crashes, buyers are now far more clued-up,” he says. “They have ready access to information and are much better at spotting what is and what isn’t a good deal – the billionaire Russian who has survived the crash will come back knowing that asking prices are 30% less than what he was asked a year ago, and will bear that in mind.”

The record of past transactions on Belgrave Square adds to the impression that the asking prices for No 10 and No 31 are optimistic. No 35, for example, has been on and off the market for almost four years as the owner tries – hitherto in vain – to get £32m for a property with just 19 years left on its lease.

So can either of them achieve three times more? The pound’s plunge against the euro and the dollar in recent months has certainly made London property more attractive to overseas buyers, but there is still the problem of those price tags.

“I know there are people around who would spend £30m-£40m on a house,” says Camilla Dell, managing director of Black Brick Property Solutions, a buying agent specialising in international purchasers. “But even they need something special in a property, and would expect it to have a private road or a large garden for that sort of money. A sale like this would completely skew everything else in the top end.” Watch this space.

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