Soon its name will join the ranks of Britain’s great stately homes. Already, in the most elite of circles, it is being whispered in awe: ‘Witanhurst. Do you know it?’

The residents of London’s Highgate certainly do, for this mammoth property has caused an ongoing row, as planners have repeatedly rejected lavish plans for its development.

The Georgian-style mansion is London’s second-largest private residence, after Buckingham Palace.But following a short-lived renaissance in 2002 as home to the BBC’s Fame Academy, it had been allowed to decay.

Now, the planning issues having been resolved, it is being turned into a modern-day Xanadu, the palatial mansion immortalized in the film Citizen Kane. The glittering 65-room palace will include 25 bedrooms and 12 bathrooms and an imperial walnut-paneled Grand Ballroom.

A vast two-levels subterranean extension will almost double its size, making room for a 70ft swimming pool, sauna, hairdressing salon, massage zone and a huge cinema suite.

Diggers are carving an enormous  cave beneath the house, which will make the property just 2,000?sq ft smaller than Buckingham Palace. Staff accommodation and a 25-space car park will complete the £50million expansion.

A borehole system, driven deep into the ground, will heat the house, using the Earth’s warmth.Documents lodged with Camden council show that the main house will remain largely the same, except for the demolition of a 20th-century wing, which will be replaced by a super-sized ‘orangery’. Work is due to be finished in autumn 2012. via