David Graham’s four-storey underground house

David Graham, a Canadian multi-millionaire, plans to triple the size of his London mansion by digging down 50 ft to create a four-storey basement complete with swimming pool, spa, ballroom and no fewer than 20 toilets.

Unluckily for David Graham – a former cable TV mogul – building under your home is no less controversial than extending it on the surface. His plans have caused fury among the well-heeled residents of Knightsbridge, including a novelist and a duchess.

Architects’ drawings show how the vast house, originally built as a school in the 19th century, will be created by excavating deeper than the height of neighboring homes.

As well as the spa area, it will have servants’ quarters consisting of five staff bedrooms. There will also be wine cellars, an art storage room, parking for three vehicles and a car lift.

In total the house will have 15 bedrooms, seven bathrooms and 20 toilets. Rooms upstairs will include a library, bar, and laundry.

Estate agents estimate the property could be worth up to £90 million ($138 million) with the work completed.

Neighbors say the massive excavation, which will involve 1,375 skipfuls of earth being removed, will bring years of “utter misery”. But David Graham says the extension is necessary to provide his “family’s needs as required by today’s contemporary living”.

The 75-year-old, who was once married to Barbara Amiel, wife of disgraced media mogul Conrad Black, bought the property in 2000 and converted it into a house.

He was granted permission to build a pool and one basement level, but now wants to add three additional storeys extending under the house and garden.

According to the application submitted to the Royal Borough of Kensington and Chelsea, David Graham “has been looking for a larger house in the locality which has proved unsuccessful”.

He therefore wishes to extend his home to provide further accommodation for his family and servants’.

The firm that filed the application on his behalf said the conversion would take up to four years.

Neighbors who have objected to the plans include novelist Edna O’Brien and the Duchess of St Albans Gillian Beauclerk.

The duchess said: “These plans are absolutely monstrous and unnecessary. It’s just absolute greed.

“No one needs that much space.”

A spokesman for the nearby Milner Street Area Residents Association said: “Why should we all suffer just so one man can indulge his fantasy?”