Is this the world's most expensive apartment?

Fancy a waterslide that takes you straight from your dance floor to your swimming-pool? What about an unobstructed view of the crystal clear Mediterranean Sea? Believe it or not, this property exists… and it could be yours for a reported $400 million.

The tiny principality of Monaco, nestled on the French Riviera, is best known as a playground of the rich and famous – with nearly one in every three residents a millionaire, according to WeathInsight research. But it could soon garner a reputation for more than its glamourous Grand Prix and numerous casinos – as it is now home to what could become the world's most expensive apartment.

The property in question is the 3,300-square-meter, quintuplex "Sky Penthouse" in the soon-to-be-completed Odeon Tower.

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Developed by Groupe Marzocco – a luxury real estate company – the double skyscraper of 70 apartments will tower over the principality at 170 meters high.

It promises future residents the "five-star experience" with a 24/7 concierge, an entertainment area (with private movie room and lounge ) and a "wellness centre", including a gym, sauna, private spas and a selection of pools.

Spread over five floors, the penthouse property will include, among other things, a kitchen on each floor (connected by a lift), five bedrooms, a living-room, dining-room, private home-cinema, sauna and a swimming pool – which is also accessible via a slide.

Scheduled to be completed in September 2015, the Marzocco family is expected to put it on the market for around 300 million euros ($400 million), potentially making it the world's most expensive apartment.

"A property like this one is bought, it is not sold", Niccolo Marzacco said in an interview with French magazine Challenges.

A record breaker?

Monaco is no stranger to "ultra-prime" properties, however. In its 2014 Monaco Residential Market report, real estate agent Savills described the city as "a rare gem of particular value in the vault of ultra-wealth".

The region is supply-constrained due to its limited size, Savills said. Its area is just 6 percent the size of Manhattan and 3 percent of Hong Kong Island, but demand for property from the world's ultra-wealthy is particularly high.

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Whether the Sky Penthouse will sell for the rumored $400 million is another matter, however. Late last year Savills warned that the billionaire residential property boom was slowing.

But for Yolanda Barnes, director of world research at Savills, Monaco is in a league of its own.

It is a "honeypot for ultra-high net-worth individuals", she told CNBC in a phone interview. With both land and available properties scarce, the principality "will have a strong market regardless of what happens to other kinds of billionaire properties," she added.

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Furthermore, this type of property is rare and "rarity is incredibly important" for the super-wealthy, Barnes added. "So too is prominence and the importance of Monaco as a location in the billionaire real estate market."

But despite a whopping price tag which could see it named the world's most expensive apartment, its large size means the penthouse might not claim the record amount per square foot. This is currently held by the owners of 15 Central Park West, New York, which sold for $13,000 per square foot earlier this year.