The founder of Foxtons estate agency is spending millions creating a five-storey extension beneath his home with room for a tennis court and vintage car museum.

Jon Hunt, who made £370million when he sold the estate agency at the height of the property boom in 2007, obtained permission in August for an extension beneath the back garden of his eight-bedroom property in Kensington Palace Gardens.

The two-floor scheme was to be more than 80ft deep and stretch 180ft into the garden. However, the 55-year-old, who has an estimated fortune of £660million, decided this month to increase the size of the development.

He now wants to excavate a further three floors beneath the house in London's most expensive street and about 65ft out under the front garden because he needs more space for his collection of classic Ferraris and other sports cars. Mr Hunt's move comes amid controversy surrounding basement extensions in London.

The number has doubled in many areas as families seek to improve their homes rather than sell in a falling market. But those living near the developments say the building work is intolerably noisy and accuse councils of failing to police the disruption.

Mr Hunt's plans are grand even by the standards of Kensington Palace Gardens. The average price of a property in the road now stands at £21,954,210 despite an 18.4 per cent fall in prices during the recession.

Mr Hunt, who is married with four children, bought the house in 2005 for £15.75 million.

Last year he is believed to have rejected an unsolicited approach for the property from Aditya Mittal, the son of steel magnate Lakshmi Mittal, who offered him £200 million.

The design statement accompanying Mr Hunt's planning application said: "The need for the additional floor space has risen through an increase in the applicant's requirements for a private museum/gallery space to display vintage cars and motorbikes."

The extension, it says, will create "an appealing museum/gallery in which to view the artefacts ... displaying the vintage cars as pieces of art". Experts have warned that huge basement extensions could pose a threat of flooding during heavy rain. They say they could prevent water draining away effectively.

Mr Hunt is not the only wealthy west London resident digging down to extend his home.

The owner of Debenham House, a 22,000 square-foot house in Addison Road, near Holland Park, recently applied for permission to build a swimming pool and car park beneath the property.

Although schemes on such a scale are rare, digging down remains the most cost-effective way of adding space in the more expensive parts of London.

Kensington and Chelsea council has approved 616 subterranean schemes in the past 27 months.

Daniel Moylan, the borough's deputy leader, said: "We don't let people build up or back, so people are going down."

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