To spend £50m on a new house seems excessive; to splurge another £30m on doing it up astonishing. But the super-prime section of the London property market is a rarefied world, with laws of its own. Indeed, the new Kazakh owners of the grandest house on The Bishops Avenue, north London’s billionaires’ row, believe they bagged a bargain when they bought Toprak Mansion earlier this month.

But the decor, it seems, is not right. And, despite the five reception rooms, nine main bedrooms and 16 bathrooms, spread over 23,000 sq ft (the equivalent of 15 semis), the place is not big enough. So, although Toprak, the most expensive new-build property sold in Britain, has probably been lived in for only a week or so since it was completed seven years ago, the owners are calling in the builders.

By the time they’ve finished, as well as all the basics de rigueur for the self-respecting billionaire – such as wireless audiovisuals, pressurised purified water on tap and automatic security shutters – they will have a 30-seat cinema, squash and tennis courts, a billiards room, a beauty salon, a 24-carat gold-plated whirlpool bath, a new staff lift and a small river, complete with canoe. Even the helicopter gets its own lift: the landing pad in the garden will sink, James Bond-style, into the ground and a roof will slide across on top.

The end result, expected to cover 42,000 sq ft, looks certain to outclass its next-door neighbour, Lakshmi Mittal, whose own 25,000 sq ft house, Summer Palace, can be peered into from the mansion’s 80ft main reception room. (Not that Mittal is often there: the Indian steel magnate spends most of his time at his other home in Kensington Palace Gardens, west London.)

The four-storey house with a neoclassical portico was built for Halis Toprak, a Turkish businessman, and has been marketed discreetly for several years. The asking price climbed from an initial £30m as the London property market soared. Agents nicknamed it Top Whack.

The new owner of this extraordinary statement of personal wealth is a diminutive 75-year-old woman, Hourieh Peramaa, who fled her native Kazakhstan at the age of 17. A year later, while living in an Iranian refugee camp with not a possession to her name, she met her future husband, a medical student from a wealthy family. She helped him acquire land and over the years has built a property portfolio worth more than £1 billion that stretches through Iran and Europe.

Peramaa, who keeps her name out of the public domain by buying through trust funds, divides her time between the south of France, Toronto and London, and has left her daughter-in-law Yassmin, 33, to coordinate the building work, which is expected to take 18 months. In March, 50 builders will rip out walls, floors, wiring, plumbing and windows. In fact, everything is going apart from the elliptical swimming pool and the double- curved staircase – and even they are being revamped. Two huge Venetian chandeliers are already on order, one for the hall, the other to hang over the pool. A new extension will house the staff quarters.

Yassmin has employed one of the “most important” interior designers in Italy, and is flying out weekly in the family jet to liaise. All internal doors are being replaced with automatic sliding jobs, some opened by fingerprint recognition. The controls are cutting-edge and wireless.

“There have been a lot of technological improvements since this house was built. Mother-of-pearl tiles, for instance, didn’t exist then,” says the impossibly glamorous Yassmin, who is a good 6ft tall, even without her 5in stilettos, and is surely a size zero.

There’s also the matter of taste. “My mother-in-law has high standards,” she says, and there’s little doubt that Toprak, as it stands, is more about grandiosity than style. Finding the right house to buy – even if you are prepared to pay up to £100m – is not easy.

The family were looking for two years, and despaired of getting what they wanted. “We thought we might buy one of the penthouses at One Hyde Park,” Yassmin says. “But they were asking £112m, it had a communal entrance and communal pools, and it was only 20,000 sq ft.

The layout was all wrong, and the walls come only in magnolia. To have any wall covering, or interior design done, was going to cost another £8m.” At less than half the price, Toprak Mansion, with its 2½ acres, was a bargain. For comparison, Yassmin points out a new 22,000 sq ft house with just one acre, about to be completed a few doors away. It is expected to go on sale for £45m to £50m.

What’s a poor billionaire to do? “This was the best she could find,” says Yassmin, who says her mother-in-law is happy to spend what it takes. “She looks at this house as an investment. It’s one of the most substantial houses in the world.” With this in mind, the first thing the family did was change the name to Royal Mansion. “Historically, royal stands for quality. We wanted it to reflect the importance of what the house was going to become, and this road has been associated with many royals over the years, from Saudi Arabia, Brunei and Greece,” Yassmin explains.

About half of the £30m renovation cost is expected to go on plumbing, automation, the pool area, the cinema and underfloor heating. Doing up the garden will cost £2m; the heli-lift £500,000. Security is a priority – while visitors will arrive at the stately front door, the owners will make use of a hidden basement entrance. “There’s no angle from which an intruder could shoot a photograph – or anything else,” Yassmin says. When the house is complete, it may become Peramaa’s London base, along with various members of her family and associates.

Equally, if another deal comes along, she may decide to sell. Should she stay, the dining table, one of the few fixtures they are keeping, is plenty big enough, as it seats 50. The many grandchildren are sure to avail themselves of the nine bedrooms – there will be another four for staff and three for security detail. The master bedroom on the first floor is 3,500 sq ft, and that’s without the enormous dressing rooms and bathrooms, two of each.

So, will prices of super-prime properties continue to soar upwards, despite the turbulence below? Trevor Abrahamson, managing director of Glentree International, the agents who sold the house, is confident they will remain stable, at least. Earlier this month, he set the previous record for a new-build property, £35m, with the sale of a nearby house called Palladio to Lev Leviev, an Israeli billionaire. “Up to about £12m or £13m, it’s mostly English people who buy in northwest London,” Abrahamson says.

“Above that, they are 80% foreign, from the Soviet Union satellite countries, India, Nigeria, some from the Middle East. They have usually made their money from oil, and have never been wealthier. London is the financial capital of the world – it is the place to buy properties and they feel they may as well get one.

“A lot of these people come from troubled areas of the world and are buying a bolt hole against anything going wrong at home.

It is also a statement of their wealth. Frankly, Toprak and Palladio are extreme statements that you have arrived.” Robert Bailey, a property buying agent who specialises in prime central London, also reports a degree of uncertainty at the topish end of the market, at the £3m to £10m level, “but when you get to £20m plus, it’s still extremely active.

“A lot of people from Russia, India and Kazakhstan are looking for trophy homes and they are all chasing a few houses,” he says. “If you are worth billions, to spend £50m is not so much, and there has been tremendous capital growth at that level. Three or four years ago, an unmodernised house in Belgrave Square might have cost £10m. Last year, a refurbished one sold for £60m.”

Yet Bailey fears there may be storm clouds ahead. Proposals by the government, mooted last year, to start chipping away at the highly favoura-ble tax status enjoyed by wealthy “nondomiciled” foreigners who live in Britain – including the introduction of a £30,000 flat rate charge – have led to suggestions that some may quit the country.

“If people start going abroad, we are going to find ourselves with a whole load of expensive houses with no buyers,” says Bailey.

None of this worries Yassmin, nor her mother-in-law, who is more than happy with her new home. This week, they will host a housewarming party for 400 guests, at which the Dom Pérignon and caviar will flow freely. They are starting as they mean to continue – no expense is to be spared at the Royal Mansion.

Money talks

- Andrew Lloyd Webber was one of the first buyers to break the £10m barrier when he bought a Belgravia townhouse in 1989

-The record was smashed in 1992, when The Holme, a palatial residence in Regent’s Park, sold for more than £20m, signalling the start of the mega-market

- In 2004, 18 and 19 Kensington Palace Gardens took the record. Lakshmi Mittal, the Indian-born steel magnate, bought the house for £57m

- Rumours of £80m sales for penthouse flats in the as yet unbuilt One Hyde Park, in Knightsbridge, have not been confirmed

- Updown Court, a new-build mansion near Windlesham, Surrey, with five swimming pools, has been on the market for £70m since 2005 – and is still without a buyer

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