Even by the lavish standards of west London’s numerous multimillionaires, Jon Hunt’s plans for a new basement went way beyond extravagant.

A resident of Kensington Palace Gardens — the most expensive street in Britain — Hunt planned a five-story basement that would house a car museum, a tennis court, an elevator, a swimming pool and a rotating Ferris wheel for the vehicles.

Citing her diplomatic rights under the Vienna Convention of 1961, Hunt’s next-door neighbor, French Ambassador Sylvie Bermann, took legal action. She lost a battle at the High Court last year but is now launching a legal challenge at the Court of Appeal.

Ambassadors from Saudi Arabia, Japan, Lebanon, Russia and India — not known for backing one another — also live on the street and have opposed the construction. They all signed a recent letter of protest sent to the Foreign Office and the Crown Estate, the property company that owns the land.

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London is often a battleground for clashes between big-splash projects and old-school sensibilities. There has been much hand-wringing over London’s changing skyline, for instance, and worthies such as Prince Charles are apt to pipe up with unsolicited opinions.

But this spat, concentrated in one of London’s swankiest neighborhoods, has property owners wrestling with a problem unique to the ultra-rich — how to build a mega-basement without infuriating the neighbors.

Hunt is not the first to cross his neighbors with such plans. The mega-basement craze here began around the turn of the millennium when it was often the most economical way to carve out extra space. With strict rules against extending aboveground and the phenomenal expense of selling up, many of the fabulously wealthy opted to simply burrow down instead.

But there is nothing simple about digging in central London, where residents live cheek by jowl and a spaghetti network of underground trains runs deep beneath the surface. Digging out London clay is hard work involving trucks, jackhammers and excavators.

Neighbors complain of unbearable noise and traffic problems that can go on for months or even years. Some are concerned about the safety of their homes.

The controversy has attracted star power over the years, including Queen guitarist Brian May and actress Joan Collins publicly deploring the disruptions caused by “iceberg” basements that render a property bigger below ground than above.

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Amanda Frame, the chairman of the Kensington Society, a residents association, said that on some streets the enthusiasm for mega-basements spread quickly.

“I call it the Ebola syndrome,” Frame said. “If one street had one, then suddenly everyone would start putting in applications, and there are streets that are just riddled with them.”

She said that Hunt’s plans, first submitted in 2008, drew attention from the wealthy, who were impressed by his bold imagination, and by the local council, which was energized to address the growing concerns triggered by mega-basements.

London’s well-heeled have used their cavernous caves to house cinemas, gyms, climbing walls, Turkish baths, bowling alleys and ballrooms. One hedge-fund manager built a four-story basement that included a 16-foot-deep swimming pool and a high-diving board.

“It’s a really big central-London problem,” said Karen Buck, the member of Parliament for Westminster North who has seen the number of approvals for basement excavations in the London borough of Westminster almost double between 2010 and 2014.

She said that most people don’t take umbrage with their neighbors extending properties per se but that it can be a tougher sell when their neighbors are seen to be building an underground lair for entertainment only.

To be sure, London’s basement wars are largely concentrated in a few select areas and in some cases involve foreigners swooping in to buy and renovate properties they rarely live in. The Evening Standard newspaper once called the London borough of Kensington and Chelsea a “ghost town of the super-rich.”

But locals who do live there have fought back, and last year new rules were implemented restricting subterranean developments. Now, property owners in the borough can build only a one-story basement in most cases, and it can extend underneath only 50 percent of the garden. Other boroughs in central London have imposed or are considering similar restrictions.

One homeowner made headlines last year after her plans for a new house with a two-story basement were rejected. She then painted the exterior of her existing house with red and white stripes in what many saw as a flamboyant snub to the neighborhood.

Hunt lives in the same borough, but his initial proposals were approved before the new restrictions took effect.

Hunt, who declined to be interviewed, is the founder of the real estate giant Foxtons and lives in a white mansion that once belonged to the Russian Consulate. His house is a short stroll from Kensington Palace, the London residence of the Duke and Duchess of Cambridge.

In a new application submitted in January, Hunt significantly scaled back his basement plans — the new drawings don’t appear to show a Ferris wheel. The borough of Kensington and Chelsea could rule on his resubmitted application later this month, but objections have already rolled in.

In a letter written last month, the Embassy of Japan wrote that Hunt’s proposals would have an “adverse impact on our diplomatic activities which require tranquility and privacy.”

The French Embassy said in email exchanges that they were “open to dialogue” but that Hunt’s scaled-down basement was “still likely to disturb our normal diplomatic activity.”

Henry Pryor, a London property expert, said he wasn’t surprised that the French were “trudging in Gallic fashion” to object to the basement extension, but he said that “if the roles were reversed they would likely be indignant that someone was suggesting what they can and cannot do with their property.”

“Whether Hunt is taking this to extremes, I’ll leave it to others to decide,” he continued. “But there is a theory here that an Englishman’s home is his castle, but it turns out, there are an awful lot of things that you can’t do despite the fact that the castle is yours.”

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