To most, the scene of water flooding into a ballroom as the floor is swallowed up by a swimming pool will only ever be the stuff of silver screen legend.

But one newly built mansion in Hampstead Garden Suburb could turn movie magic into reality for anyone willing to match the �30million asking price.

A vast swathe of the two-tier chequered-floor basement in Winnington Road can open up in a matter of minutes, giving way to a swimming pool in the style of the Christmas classic: It’s a Wonderful Life.

But the feature is “not uncommon”, according to Hampstead estate agents Savills.

A property in neighbouring Bishop’s Avenue – better known as millionaire’s row – has also installed, albeit, a smaller version.

Sales director Simon Edwards said: “It’s becoming more common. There is a company that specialises in building them, but only at the absolute top end of the market.

“If people have enough space they don’t need this feature, but if you’re more constrained it gives you two different rooms. It also helps insulate the pool as well – you don’t need a cover.”

The eight-bedroom mansion, a stone’s throw from a �10million home linked to the Gadaffi, also houses a cinema, leisure complex and a plunge pool in the garden.

The �30million mansion, backing onto Hampstead Golf Course, is on the border of the Suburb conservation area, sitting next to other palatial detached homes.

A Trust spokeswoman said: “The Trust has a say over the external arrangements of the house, but not the internal arrangements. They are large houses on large plots of land.”