Luxury property developer Christian Candy has sold the former head office of his business empire – now refurbished into an opulent five-bedroom home complete with an underground swimming pool, massage room, aquarium and cinema – for £63m.

Rutland House, a six-storey Victorian terrace in Knightsbridge – a stone’s throw from Harrods and One Hyde Park, the uber-luxe enclave developed with his brother, Nick – was the most expensive property to change hands in June, a month when the global super-wealthy spent more than £407m on London properties worth more than £10m each.

The deal capped the biggest month for sales of £10m-plus houses in the capital since the government nearly doubled the stamp duty charge levied on mega-mansions, according to research by the estate agent Knight Frank and data service LonRes. The buyer would have been landed with a £7.4m bill for stamp duty, because the tax on properties that cost more than £1.5m was increased from 7% to 12% in 2015.

The £63m sale crystallises a huge profit for Candy, who bought the property for £18m in 2007 and combined it with an adjacent summer house bought for £4m.

One Hyde Park apartments in Knightsbridge, London. Photograph: Alamy

Prime London property prices, especially for new-build high-rise apartments, have been falling since the stamp duty increase in April and the Brexit vote. According to property research specialists Molior London, developers have been selling at discounts of up to 30% to corporate buy-to-rent investors who buy in bulk. Molior research shows that 39% of new-build sales were these bulk deals in the three months to June.

But at the very top of London’s so-called “super-prime” property market – properties valued at more than £10m – business has picked up. The number of prospective buyers registering interest in that top price bracket was up 7% on 2017 levels in the first three months of this year.

Liam Bailey, Knight Frank’s head of research, said because sterling was relatively weak against other currencies, foreign buyers could make savings, with 60% of super-prime buyers coming from overseas, especially Europe, the Middle East, Russia and India.

Marcus Dixon, LonRes’s head of research, said super-prime sales were picking up because prices have fallen by as much as 25% from their peak in 2014 and “people are realising that, while these sort of properties are hugely expensive, they are much better value than they were”.

Dixon said the stamp duty hike to 12% had initially put off buyers “but now they are just factoring it into the price and realising the overall cost is much lower than it was”.

Rutland House, the five-bedroom luxury house in Knightsbridge recently sold by Christian Candy for £63m. Photograph: Google

Henry Pryor, an independent luxury-property buying agent, said the market had stalled on houses valued at £2m to £4m because of concerns about the effect of Brexit. “But, at the higher level, people have other challenges and are not vexed by Brexit.

“When it comes to the super-rich, there is much more understanding of mortality,” Pryor said. “It’s all very well having all this money, but you can’t put your life on hold if you’re a billionaire and you’re 67. If they have £30m to drop and they like the house, they’ll buy it.”

The buyer of Rutland House is so far unknown. Savills, the estate agency that sold the property, confirmed the sale price but declined to say who would be moving into the 13,616 sq ft property (an average new UK three-bed home is about 1,000 sq ft). The sale has not yet been recorded on the Land Registry.

The house is on a private road once home to the ballerina Margot Fonteyn and the cellist Jacqueline du Pré, with a gatehouse staffed by security guards. One of the guards said: “We know it’s been sold but we have no idea who to. It is very much on a need-to-know basis.”

Whoever the owner is, they will no doubt lead an ultra-luxe lifestyle. Sangeeta Goyal, the interior designer commissioned by Candy to work on the house, was asked to create “a single, luxurious private residence for a very wealthy and busy family ... with the discreet use of the latest in technology and with a very high-end and bespoke design”.

She said on her blog: “There is an embassadorial [sic] feel ... I used neutral colours to create a calming effect. The walls are covered in pearl-effect polish plaster to give the room a slight shimmer. The aquarium passes through the wall into the dining room on the other side. The rug is of wool with silk design which catch [sic] the light and glisten. A mother-of-pearl panel in a herring- bone design frames the aquarium. The coral accent makes the room pop and stimulate [sic] the mind to encourage conversation.”