The couple have three children, and divide their time between London and homes in Geneva and Gstaad.
The extra space will add a subterranean dining room, cinema, kitchen and gym — updating the Victorian property into what the architect who designed the project describes as a “modern family home with appropriate facilities”.
The report by the project architects, Boundary Space, added: “The proposals reflect that the client’s family has grown, and the accommodation is no longer conducive to modern family living.”
Although several homes on the mews already have basements, the Bertarellis would be the first to excavate to two storeys, and several of their neighbours have instructed lawyers to fight the proposal.
They are backed by local councillor Rachael Robathan. “The size of the extension is disproportionate,” she said.
“There have been a large number of objections from other residents within the mews,” said John Walker, the council’s operational director of development planning, in a report that will be considered by the council’s planning committee this week.
“Concern has been raised by residents that the use of the basements has not been specified, and that they might be used as recording studios. They are concerned that the excavation of new basements is a risky construction process, with potential harm to the property and adjoining buildings.”
According to Land Registry records, Bertarelli purchased the house in 2000 for just over £5.5 million.
Given property price rises in the area, it could today be worth in excess of £10 million.
Ironically, if the couple lived a just few streets away, their home improvement plans would be impossible since Kensington and Chelsea council has introduced a ban on “excessive” two-storey basement extensions, which it says are unacceptably disruptive to neighbours.
Earlier in March, it was confirmed that Westminster Council is to introduce a similar policy to limit the size of basement extensions by the end of this year.
Mr and Mrs Bertarelli were unavailable to comment.
Westminster Council approved the Bertarellis’ application at a meeting on the evening of 11 March.