Beneath the games room of a £15 million ($30.4 million NZD) house in the UK's Hampstead is the concrete shell of a swimming pool that the current owners have never fully installed.

They had it built four years ago, at the same time as the basement gym, car showroom and cinema, in case a future buyer insisted on having somewhere to take a dip – a good call, given that around 400 mansions in London now have subterranean pools.

Yet there's a chance that this pool could remain permanently hidden beneath temporary flooring.

New analysis of UK council data by law firm Boodle Hatfield suggests that the trend for"iceberg" homes – those with lavish, mega-storey basement extensions that contain underground banqueting halls or car museums, and are often significantly larger than the actual house above ground – is on the wane. Applications for basement excavations in Westminster fell by 27 per cent last year – 99 in 2019, compared with 136 in 2018.

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"Buyers are becoming less excited about amenities that require maintenance and can easily go wrong," says Marc Schneiderman of north London prime estate agents, Arlington Residential. "In some ambassadorial houses, there are plant rooms of more than 3,000 sq ft. You need a manager to run it."

According to developer Mike Spink, whose clients are some of the wealthiest people in the world, when it comes to decor, the super rich's priorities are changing. "I'm seeing more restraint and common sense. The home is a place to recharge. My clients want tranquillity and things that work."

Spink mainly works on grand, period townhouses and, while he still builds palatial basements, he insists that they are in proportion to the size and scale of the house. A recent project in Pembridge Square, Notting Hill, had planning permission for a four-storey basement yet, he ended up developing just one level. "It's not about quantity of space any more," he says, "it's about quality."

What the super rich are really splashing out on.

Super security

This is, by far, the biggest focus of a super-wealthy homeowner today, according to Guy Bradshaw, head of London residential at UK Sotheby's International. Bradshaw recently had a buyer who would only consider properties with a wall out the front and bullet-proof glass.

The super-rich also like their mansions to have "winter access", adds Gorrell, a covered underground access point where they can enter the house discreetly by car. Full-time security staff who operate as doormen during the day have also become the norm, and with so much run by wireless network, homes must be fortified against cyber attack. "They need a separate networks for guests, door entry systems and cameras," Gorrell says. Thermal cameras are also replacing CCTV with virtual alarmed trip lights across boundaries.

State-of-the-art energy systems

Until recently, only one per cent of super-rich buyers enquire about a house's energy credentials, says Spink, but the attitude is changing. "No matter how much money you've got, you don't want to be throwing it away," agrees Gorrell, whose business is increasingly being commissioned to integrate renewable green systems into mega mansions. "If you've got five homes around the world, you're going to be looking at consumption. There's a growing moral conscience." Gorrell uses car battery technology to run heating and electrics via solar cells, and air source pumps that can extracts heat from outdoor air, even when it's -10C.

Scuba storage

Increasingly, basements are returning to their traditional usage: storage. In one of Spink's houses, he installed a Big Yellow-style storage corridor, with separate rooms for luggage, scuba equipment, riding kit and winter coats, alongside a safe room, with tables for sorting valuables. "It made packing so easy for their staff – they could go to each room and get out exactly what was needed," he says.

Simpler tech

In the past, super-rich buyers were turned off homes that didn't have complicated lighting systems, but now simple is best. "I had a client recently who was closing the curtains whenever he pressed the light switch," Schneiderman says, "it was driving him mad." The lighting, heating and sound systems that Gorrell installs in super-rich homes are operated by no more than four intuitive single-press switches.

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"These are the world's most sophisticated electronics hidden behind a beautiful and incredibly simple interface," he says. "And speakers are plastered into walls and ceilings, with music 'pushed' into around the house from your smartphone."

Serveries

Along with a designer kitchen, with two islands (one for the chef, and one for you to sit with a glass of champagne), super-mansions increasingly have a smaller, more functional, hidden kitchen, away from the living space, where the chef and kitchen staff can get on with their prep. "It ensures the food has a more seamless appearance," Gorrell says.

The entertaining space is also now a cross between a dining room and a sitting room, with furniture designed for relaxing over long periods, while being served tapas-style dishes.

The gym suite

Not just one gym, but a series of light-filled studios with sprung floors, dedicated to free weights, Pilates, techno gym, yoga and meditation – some even have beauty treatment rooms and hairdressing salons and Formula 1 car simulators. Surge pools and snow caves, which produce real snow for muscle recovery, are also popular as well as saunas and whirlpools.

No bling zones

Gold, marble and diamante finishes are being replaced by calm, understated, tasteful interiors. "We live in an age of information overload so buyers want their homes to be a calming, relaxing space without too many images, buttons and colours," Gorrell says.

Instead, top architects are focusing on simple, bespoke features – a sculpted staircase, for example, or a nine-metre aquarium – and creating bedroom suites with dressing rooms and bathrooms for each family member. Pools and car museums are a sideshow, Spink says.