The house’s owners have built a vast basement that amounts to an underground village. Illustration by Michael Kirkham

Witanhurst, London’s largest private house, was built between 1913 and 1920 on an eleven-acre plot in Highgate, a wealthy hilltop neighborhood north of the city center. First owned by Arthur Crosfield, an English soap magnate, the mansion was designed in the Queen Anne style and contained twenty-five bedrooms, a seventy-foot-long ballroom, and a glass rotunda; the views from its gardens, over Hampstead Heath and across the capital, were among the loveliest in London. For decades, parties at Witanhurst attracted potentates and royals—including, in 1951, Elizabeth, the future Queen.

In May, 2008, I toured Witanhurst with a real-estate agent. There had been no parties there for half a century, and the house had not been occupied regularly since the seventies. The interiors were ravaged: water had leaked through holes in the roof, and, upstairs, the brittle floorboards cracked under our footsteps. The scale of the building lent it a vestigial grandeur, but it felt desolate and Ozymandian. A few weeks later, Witanhurst was sold for fifty million pounds, to a shell company named Safran Holdings Limited, registered in the British Virgin Islands. No further information about the buyers was forthcoming.

In June, 2010, the local council approved plans to redevelop the house and five and a half acres of grounds, maintaining Witanhurst as a “family home.” It was the culmination of a long battle with other Highgate residents, who did not welcome such an ambitious project. Since then, Witanhurst’s old service wing has been demolished and replaced with the so-called Orangery—a three-story Georgian villa designed for “everyday family accommodation.” And beneath the forecourt, in front of the main house, the new owners have built what amounts to an underground village—a basement of more than forty thousand square feet. This basement, which is connected to the Orangery, includes a seventy-foot-long swimming pool, a cinema with a mezzanine, massage rooms, a sauna, a gym, staff quarters, and parking spaces for twenty-five cars. In late 2013, the local council approved plans for a second basement, beneath the gatehouse, which will connect that building to both the main house and the Orangery. Earlier this year, the owners also sought planning permission to extend an underground “servants’ passage.”

When the refurbishment is complete, Witanhurst will have about ninety thousand square feet of interior space, making it the second-largest mansion in the city, after Buckingham Palace. It will likely become the most expensive house in London. In 2006, the Qatari royal family bought Dudley House, on Park Lane, for about forty million pounds; after a renovation, its estimated resale value is two hundred and fifty million pounds. Real-estate agents expect that the completed Witanhurst will be worth three hundred million pounds—about four hundred and fifty million dollars.

If a vast and lavishly appointed house in Manhattan—a palace nearly double the size of the White House—were being redeveloped on the edge of Central Park, New Yorkers would want to know who lived there. Londoners are equally inquisitive, and concerted efforts have been made to uncover the identity of Witanhurst’s owners. Shortly after the house was sold, it became known—from local gossip and publicly accessible planning documents—that Witanhurst belonged to a family from Russia. Several newspapers speculated that the owner was Yelena Baturina, Russia’s richest woman, and the wife of Yury Luzhkov, then the mayor of Moscow. (Luzhkov and Baturina reportedly enriched themselves while he was in office, before Luzhkov clashed with the Russian government; she now lives in London.) Baturina denied owning Witanhurst, and in 2011 she sued the London Sunday Times for publishing an article titled “BUNKER BILLIONAIRESS DIGS DEEP.”

The Baturina lawsuit and the continued secrecy surrounding Witanhurst have intensified the guessing game. Generally, the names of homeowners in Britain are listed in the Land Registry, which can be read for a small fee. But listings for properties owned by offshore companies do not disclose individual beneficiaries. In the British Virgin Islands, records reveal merely the name of the “registered agent” of Safran Holdings—Equity Trust Limited, a local agency that holds several such positions and is connected to the company by name only—and the company’s post-office box, on the island of Tortola.

A recent investigation by the Financial Times found that more than a hundred billion pounds’ worth of real estate in England and Wales is owned by offshore companies. London properties account for two-thirds of that amount. Charles Moore, a former editor of the Telegraph, says that London’s property market has become “a form of legalized international money laundering.” For Highgate residents, however, worries about the lack of transparency in the purchase of Witanhurst have come second to a more English concern. People irritated by the construction noise and the traffic that have blighted their normally quiet neighborhood have no owner to complain to—only managers.

It might have been expected that the identity of Witanhurst’s owners would slip out, given the volume and the scale of work at the site, and the number of contractors involved. (Last year, I met a craftsman who said that he was on one of six teams of carpenters working there.) But few employees are told the owners’ names. Senior contractors who have dealings with the family or their agents have been required to sign confidentiality agreements. Guards protect the property, and cameras monitor the grounds. A woman who knows the owners advised me to “choose another story.” Stephen Lindsay, one of the real-estate agents who sold the house, spoke to me only after I agreed to leave my phone and bag in another room. He then put the family’s lawyer on speakerphone and announced that he would take the secret of Witanhurst’s ownership “to the grave.”

Witanhurst has always been an attractive place to park new money. In 1814, Joseph Crosfield, a twenty-one-year-old chemist, started a soap-and-candles business in Warrington, in the industrial northwest of England. The company was passed down to his son, George, and then to his grandson Arthur. By then, the company—now named Crosfield & Sons—was worth a fortune.

Arthur was no mere businessman: he was the 1905 amateur golf champion of France and a musician who composed many pieces for piano. In 1906, he won the parliamentary seat of Warrington, becoming a Liberal M.P. for four years. He eventually married Domini Elliadi, a fine tennis player who was the daughter of a Greek merchant. (She competed at Wimbledon and once won the Swiss ladies’ title.) In 1911, Arthur sold the family business. With the proceeds, he bought Parkfield, an elegant eighteenth-century house in Highgate.

Highgate had only recently been subsumed by London’s late-Victorian sprawl. Before then, the village of Highgate had abutted the Bishop of London’s hunting grounds to the north and Hampstead Heath to the southwest, and was situated on the main road from London to the North of England. (The notorious highwayman Dick Turpin hid out in a pub, The Flask, which faces Witanhurst.) Because of its elevation and its handsome views, Highgate became a favored place for rich families from London to build summer retreats.

The Crosfields knocked down much of Parkfield and started afresh with a new house and a new name, Witanhurst, which, in Old English, means “Parliament on the Hill.” Arthur commissioned the Scottish architect George Hubbard to design a gigantic new building, and the landscape architect Harold Peto to create a raked Italianate garden. Witanhurst’s décor was formal—walnut wall panels, cornices embellished with gold leaf—but it had some arriviste quirks. In the music room, three ornate crystal chandeliers were reflected in ceiling-height mirrors, giving the effect of endless dazzling light. The house also had three hundred and sixty-five windows—one for each day of the year. Nearly sixty years after the site was sold to the Crosfields, Lady Phyllis Greig, who had grown up at Parkfield, told an interviewer that she was horrified by Witanhurst. “We tried to keep things in scale,” she said. “Not like the present monster.”