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The billionaire founder of Foxtons estate agency today won a High Court battle over his plans to build a mega basement next to the French embassy.

Jon Hunt and his wife Lois had already begun work at their Grade II listed home in Kensington Palace Gardens, the ex-Soviet mission, when France’s government objected.

He aims to turn the basement into a museum for his vintage car collection. Today’s judgment effectively gives him the go-ahead to continue.

The Hunts were first granted planning permission and listed building consent in 2008 and a smaller scheme received listed building consent in 2010.

In 2011 freehold owner Crown Estates granted a limited licence to allow certain works of excavation to go ahead. But the French government challenged the validity of certificates of lawfulness granted by Kensington and Chelsea council last April to allow completion of the property tycoon’s project.

Today Mr Justice Holgate rejected five of the six French objections.

Interactive graphic: Jon Hunt's master plan

He granted only one ground which the council accepted had been “an administrative error” on its part and which has already been rectified.

The judge said “the main object” of the French claim was rejected and the error had “no continuing practical significance”. The French government must now pay all the legal costs apart from £1,500 for its success in the single ground.

In court earlier this year Mr Robert Griffiths, QC, for the French authorities, had argued the listed building and lawful development certificates were both flawed for a number of reasons and should be quashed.

He told the judge there was a lack of consultation and the excavation works already done did not amount to commencement of development but were works of repair and refurbishment under the Hunts’ lease.

If the French had won the case, the couple would have been forced into rethinking their extensive plans.

They would have had to make fresh applications to finish the work — under stricter conditions now being imposed by the local planning authority over basement developments.

Paul Brown, QC, for the Hunts, told the judge any technical “breach” of the regulations was now cured and there was no real case against them. He said of the Hunts: “They are acting lawfully. They have permission.”

In 2007 Mr Hunt sold the Foxtons chain for £370 million weeks before the credit crunch hit.