See inside a $42 million mansion in the neighborhood where London's billionaires live

This $42 million mansion's coolest secrets are hidden underground

Located in the ultra-luxurious and exclusive London neighborhood of Mayfair is "Culross House," one of the most expensive homes currently on the market in the U.K., listed for £32.5 million ($41.8 million).

"It's 8,100 square feet of pure luxury — something that you could move in and don't need to go out, " says property developer Kam Babaee, CEO of K10 Group. "All you need is a front door key."

Culross House
Knight Frank / Grant Frazer

Speaking of the front door, Babaee says his inspiration for the home's facade came from 10 Downing Street, the headquarters of the British government.

Culross House
Knight Frank

"Having been lucky enough to be invited to Downing Street on a couple of occasions, I wanted to have a door, which is as inspiring and is recognized throughout the world for this gloss black," Babaee tells CNBC. "We've achieved a door which I think is superior to the 10 Downing Street, because of the quality and the English workmanship that has gone into it."

And the $40,000 custom front door is just the beginning.

Take a look inside.

A marble-floored entrance hallway leads to the property's 30-foot-wide reception room which overlooks the garden out back.

Entrance hallway
Knight Frank / Grant Frazer
Reception room
Knight Frank / Grant Frazer

"It's very rare to get outside space," says Babaee. "We've managed to get a beautiful garden for the summer days we have in London."

Knight Frank / Grant Frazer

On the first floor of the residence are two master bedroom suites, fitted with Hermes and Lalique finishes.

Master bedroom
Knight Frank / Grant Frazer

The second floor has three additional bedroom suites — each with their own unique style.

Knight Frank

A ride down a $300,000 custom glass elevator reveals three additional levels hidden underground. "London is very valuable, very much like New York. You can't go sideways. You can't go up. So the only way to go is down" Babaee says, since space is at a premium.

Glass elevator
Knight Frank / Grant Frazer

On the first subterranean level is the home's modern gym and spa.

Knight Frank
Knight Frank / Grant Frazer

Two levels below the ground floor is the marble-covered pool as well as a giant steam room with a colorful light display.

Knight Frank
Steam room boasts a colorful light display

The posh home also has a club room with a private movie theater and cocktail bar.

Club room
Knight Frank / Grant Frazer

"The whole house is a smart house. You've got the curtains controlled on iPad, your music system, your television," says Babaee.

The smart palace also comes with high end neighbors.

"Mayfair is the No. 1 spot to live in in London — if you want to live in London," Babaee says. "At the end of the road, you have the former American Embassy, which recently got sold [for £250 million, roughly $400 million at the time] and is going to become a seven-star hotel. "

The lucky buyer will be also be neighbors with Bahraini princesses — enough to make the $42 million asking price seem like a royal bargain.

Don't miss: Oprah Winfrey just bought a $8.3 million mansion on an island near Seattle — take a look inside

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