They trace their lineage back almost 1,000 years to a relative of William the Conqueror, who invaded England from Normandy in 1066. Their initial wealth was accumulated through mines and minerals, but they owe their modern fortune to a 17th century marriage. Sir Thomas Grosvenor received 500 acres of swamp and orchard to the west of the City of London as a dowry from the parents of his 12-year-old bride.

Mayfair and Belgravia are now hotspots for some of London’s most well-heeled tenants, including luxury retailers, art galleries and hedge funds.

Grosvenor Group has expanded to 60 cities worldwide and managed assets totaling 12.3 billion pounds ($15.9 billion) at the end of last year. But the company’s core is still London.

Hugh became the head of his family’s estate when his father Gerald died from a heart attack at 64. According to the will, the sixth Duke of Westminster left 616 million pounds after debts and liabilities, with 20,000 pounds for each of his three daughters, who may receive additional income through the Grosvenor family trusts.

Gerald’s guns, fishing equipment and cars passed to Hugh, who also received his title. U.K. laws effectively limit that part of inheritance to males. That gave Hugh a personal fortune now estimated at $11.8 billion, according to the Bloomberg Billionaires Index.

Hugh had a very different upbringing than his father, who attended Harrow School and the Royal Military Academy Sandhurst. Gerald loved fast cars, women and champagne at breakfast, according to “The Reluctant Billionaire,” a 2018 book by Tom Quinn.

The current duke was educated at Ellesmere College, a private school near the Welsh border, and graduated from the University of Newcastle six years ago with a degree in Countryside Management.

Hugh then joined the Grosvenor Estate’s graduate program, working in each segment of his family’s holdings, which also includes rural estates in the U.K. and Spain. Today, as Duke, Hugh is a trustee of the Grosvenor Estate. Grosvenor Group’s CEO Mark Preston has ultimate responsibility though for the fortune of Britain’s richest landlord.

At the heart of it all is Grosvenor Square, a leafy public space in London bordered by embassies, celebrity-chef restaurants and hotel suites costing more than $5,000 a night. This month, silk roses dot the square, adding a festive touch to the center of Mayfair.

That attention to detail hasn’t gone unnoticed by local residents.