A price hike from £30m three years ago to a heady £50m in recent weeks makes Toprak — with its columns, curious green roof, and parking for 20 cars — one of the most expensive houses on sale in Britain.

For years planners have allowed some of the richest men in the world to build and buy trophy homes on the mile-long avenue that runs from East Finchley to Highgate.

Current owners include the Sultan of Brunei; King Fahd of Saudi Arabia; Lakshmi Mittal, the steel magnate and the wealthiest man in Britain, who lives in a house called Summer Palace; and Mittal’s neighbour, Halis Toprak, the Turkish industrialist and banker for whom Toprak Mansion was built.

But what made Toprak suddenly decide that his London pad was worth so much? According to Trevor Abrahmsohn, of selling agent Glentree International, the new value simply “reflects comparable prices across London” for a select club of super-size houses. Never mind that the highest price paid so far for a house in the avenue is only £15m. Abrahmsohn cites 18-19 Kensington Palace Gardens, bought by Mittal in 2003 for £57m — a price nearly three times more than the previous record for the road — as a better parallel.

In the world of the super-rich, it seems a London house’s value depends on how much a billionaire wants it. The way Abrahmsohn tells it, Toprak Mansion’s price rise came about because a wealthy east European buyer asked what Toprak would sell it for.

“He quoted £50m,” says Abrahmsohn. “The buyer is considering his position. But there aren’t too many choices out there in London ... it did take seven years to build the bloody thing.”

And the mansion is indeed, like many of its neighbours, unique, even if not to everybody’s taste. Two acres of land, an underground driveway, a dramatic eight-person glass lift soaring through the floors, a swimming pool crossed by a glass bridge, a Turkish bath to seat 20, four kitchens, a cinema and a 75ft drawing room.

Of course, neither the sale nor the soaring price tag have anything to do with the difficulties of Toprak’s business empire, says Abrahmsohn. In 2001 Toprak’s bank, Toprakbank, the 20th largest in Turkey, was placed in receivership by the Turkish government. Earlier this year his luxury yacht was confiscated.

Abrahmsohn, however, is resolutely upbeat. “Turkey has had its difficulties and Mr Toprak has had his difficulties in Turkey,” he says. “But that problem is over. A good deal of it has been solved.”

Even as long ago as the 1930s, The Bishops Avenue was nicknamed “millionaires’ row”. Residents included Billy Butlin, the founder of the holiday camp empire, and Gracie Fields, the Rochdale-born Hollywood star. Tycoons built houses there to confirm their status.

Up to the 1980s and 1990s, properties built on the avenue were extraordinarily ostentatious, many with Asian and Arabic architectural influences. The council, Barnet, which approved them, admits “they reflect the whim of their owners”. But change is afoot. There’s a growing sense that the days of building mega houses may be over. Abrahmsohn says planners have tightened up on development in the avenue, which is in a conservation area. More discreet properties are being given the go-ahead.

In fact, the latest planning row isn’t over another bling house but over proposals to develop some sites as blocks of flats, including one next door to Mittal’s Summer Palace.

Only one development of flats has been approved so far, on the site of a former retirement home, Heinrich Stahl House. But local residents needn’t worry that the flats will take the road downmarket. Prices and details haven’t been released but, according to Abrahmsohn, they’ll range from 2,500sq ft to 5,000sq ft. If they sell for the local going rate of about £1,000 per square foot, that’ll be between £2.5m and £5m each — unlikely to attract riff-raff.

Toprak Mansion is for sale for £50m with Glentree, 020 8458 7311, www.glentree.co.uk

Additional reporting by Sian Griffiths

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