Klas Nilsson is sitting in the sunlit, fourth-floor apartment of The Lancasters development on the fringe of London's Hyde Park. It is the architect and property developer's latest project to turn historic but dilapidated buildings into ultra-prime homes. Surrounding him are fur rugs, crystal ornaments and books, while running along the top of the ceiling is a re-working of the 19th-century building's original cornice. Though he is Swedish, Mr. Nilsson, 71, specializes in designing for what he sees as "the paradox of London": an old city filled with a new generation of super-wealthy people. Mr. Nilsson ignores the transience and makes homes that look permanent inside, refashioning historic outfits to the minutest detail and creating sanctuaries for the world's most deep-pocketed property buyers. The Lancasters' apartments on Bayswater Road range in price from £900,000 for a studio to £16 million for a four bedroom penthouse, boasting 24-hour concierge, valet parking, spa, swimming pool and gym, along with palm trees in the private front gardens that bring a touch of the Cote d'Azur to the opposing oaks in Hyde Park.  Mr. Nilsson's attention to detail is extreme. Sharply dressed in a suit and dark green tie and sporting a shock of white hair, he insists that everything in the showroom, from the tiniest whiff of a drain in the sparkling bathroom to the electrics, be perfect. There are no light switches—he calls them "wall acne" —but small control panels by each door for lighting and heating. "Sophisticated simplicity," he says in a Swedish accent clipped with the Queen's English.  Vision for the past: Klas Nilsson specializes in finding dilapidated buildings in unfashionable parts of London and meticulously transforming them into super-prime properties.  With the renovations completed in fall 2011, Northacre —the high-end property developer of which Mr. Nilsson is chairman and co-founder—has now sold 63 of the development's 77 apartments, raking in sales of £370 million. On top of building costs of roughly £115 million and the £67.2 million original price tag for the property, Northacre and its financial backers seem to have comfortably made a profit. They expect to sell all apartments by the end of this year and have people living in half of them.  Upper floors offer stunning views of Hyde Park and, if you squint, Mr. Nilsson's arch rival. Just beyond the tops of bare winter trees is One Hyde Park, a much-hyped prime tower development offering some of the world's most expensive apartments. While One Hyde Park was built from the ground up, is modern and ensconced in bustling, fashionable Knightsbridge, The Lancasters are quiet and regal. They're also "north of the park". In other words, off the super-prime map. But Nilsson says "unexplored" parts of London are his specialty.  After studying architecture in Munich, Mr. Nilsson first moved to England in 1971. He opened his own practice, paying the bills by carrying out architectural surveys for other developers. Itching for more, he put his pencils and rulers aside, got some money together and bought a handful of small houses in Kensington and Chelsea. At the time, they cost him around £10,000 each. He supervised the refurbishment, coming to the site every week with a wad of cash to pay his tradesmen and carpenters.  As values increased, he took on a business partner and backing in 1983 from a Swedish textile tycoon. He then created Northacre, combining architects, property managers and interior designers under one roof. Soon he was tackling more ambitious projects: Refurbishing a derelict Victorian street of brick-terraced houses in Kensington, or a Georgian terrace of 25 houses in the same area, always creating ultra-luxury homes. It turned out wealthy buyers were happy to pay a premium for historic detail. "When [other developers] had a listed building they did the minimum," he says. "We did the maximum. We tried to replace it exactly as it was."  The Lancasters development on the fringes of Hyde Park is his latest venture.  Bigger investors soon came on board for different projects, with Sweden's Ikea billionaire, Ingvar Kamprad, funding Northacre's restoration of the Queen Elizabeth College in Kensington. Mr. Nilsson refurbished old hospitals, government buildings, even insurance companies. Then he found a block of enormous, white-stucco town houses in Lancaster Gate that had served as a hotel since the 1970s and as 15 ambassadorial homes in the 1850s. Crumbling from disrepair, they were a final, undeveloped piece in the line of properties bordering Hyde Park. When Thistle Hotel put the buildings up for auction, Northacre, with property investor Minerva, put in a bid. The Candy Brothers made an offer too, but team Nilsson's was higher, and won. There were no hard feelings. Mr. Nilsson met Nick Candy, the older of the two Candy brothers, once about a year ago.  "He invited me for coffee," Mr. Nilsson remembers. He laughs, then pauses. "I like him," he concludes. Though they are rivals, Mr. Nilsson and the Candy brothers have indirectly helped one another, Northacre paving the way for super-prime renovation projects, Candy & Candy raising ultra-prime prices in the city overall.  Mr. Nilsson gave a team of his architects nine months to research the Lancaster Gate development. He discovered the block's "back had been broken" in the 1970s, leading to cracks in the partitioning walls. This meant he couldn't renovate the insides and so the whole, Grade II-listed terrace had to be gutted. With support from the local council, Mr. Nilsson finally got permission to completely rebuild the interior to its original style.  To date, most of the development's luxurious apartments have been sold, despite the economic gloom.  Then came another challenge: In summer 2008, global credit markets went into free-fall. By then, Northacre was in the middle of demolishing the buildings' interior and repairing the stucco shell. As banks tightened lending, Mr. Nilsson's project looked badly timed. But this was where years of previous developments proved useful. Mr. Nilsson had three or four loyal clients consistently investing in Northacre buildings. When he invited 10 parties to the Lancasters' dusty construction site to buy apartments off plan, two previous clients bought four buildings, or 26 apartments, for £100 million in total.  "Lock, stock and barrel, it was all funded," says Mr. Nilsson. By summer 2010, Northacre had sold half its apartments—including three buildings to a single Middle Eastern buyer for another £100 million. Today the choice for ultra-wealthy property buyers is between One Hyde Park and the Lancasters, each having benefitted from a lack of supply. "The crisis had helped," Mr. Nilsson says matter-of-factly, "because there wasn't as much competition". He doesn't seem too worried about the current economy, either." London will always be a magnet for people," he says. Already, this most energetic of 71-year-olds is investigating sites in Bloomsbury and the Strand for his next big project. "It's what we've always done," he says with a gleam in his eye. "We always go to uncharted territories."
Klas Nilsson started his career as a jobbing architectural surveyor. Today, the entrepreneurial Swede is the creator of some of the most luxurious, and expensive, super-prime real-estate in London.