A block from Central Park on the Upper East Side of Manhattan sits a majestic limestone townhouse with graceful French balconies that look transported straight from the Gilded Age. Indeed, if anyone were capable of time travel, it would be the house's owner, interior designer Juan Pablo Molyneux. An unapologetic maximalist, Molyneux is the man to see about, say, a gut renovation of your 17th-century castle or a blueprint for your private island villa—dream homes, in other words. When he overhauled several rooms in the Parisian social club Cercle de l'Union Interalliée in 2004, he installed marble consoles so enormous, they had to be lifted through the windows with a crane; more recently he created every inch of Sheikh Mohamed Bin Suhaim Al-Thani's 40,000-square-foot palace in Qatar (its pillared facade has earned comparisons to the White House).

Molyneux and his wife of 38 years, Pilar—who met as teen agers in their hometown of Santiago, Chile—moved into the Beaux Arts townhouse in 1986, when it was divided into separate apartments. They gradually acquired every unit, renovating as they went, but the residence reached its current glory as a single-family home only in the past five years, after Molyneux relocated his New York office from the first two floors and into a nearby building. Today the seven-story structure is a monument to the designer's hallmark opulence. Enter its wrought iron front gates and you'll find a sweeping staircase encased in wood paneling that portrays a 17th-century battle between the Chinese army and Portuguese invaders. Various hidden doors pop open to reveal a gleaming powder room, an old-fashioned elevator, and (most excitingly) a butler bearing tea sandwiches. Although the house is never quite "complete" in Molyneux's eyes—last year the couple added a pantry to the rooftop terrace for dining alfresco—it satisfies him, which is saying a lot. "The making of a room, in my mind, is instant," he says. "What takes a long time is finding the right guy who's going to do this kind of embroidery or make that kind of lamp so that it matches the picture in my head."

If there's one thing Molyneux's lavish interiors have in common, it's a whiff of fantasy—quite literally, in some cases. In the corner of one sitting room towers an antique narwhal tusk, its tip nearly piercing the 11-foot ceiling. "In the 14th century, people thought these were unicorn horns," Molyneux explains. "I found it at Partridge's in London many years ago. But back then you could not just call UPS and have them send it, so I prepared a box out of tubes and carried it through the airport and onto the plane with me." He laughs. "Imagine the security!"

For all his bold flourishes, Molyneux—who has thick silver hair, impeccable posture, and looks like he was born wearing trim Italian suits—approaches his spaces with discipline. "I believe in balance," he says. "I was always fascinated by a sort of casual harmony." In the second-floor parlor, a circular bronze sculpture by the Italian artist Bruno Romeda functions as a modern counterpoint to the checkerboard marble floors ("I don't like to put down carpets") and velvet-upholstered walls.

The designer is presently working on several projects abroad—including a theater in Moscow, for which he commissioned giant silk wall hanging based on the abstract works of the Russian Constructivist Kazimir Malevich—and admits that his travel schedule may be the reason he hasn't changed much around the townhouse for almost a year. "But in my world, nothing is ever finished," Molyneux says. "I am constantly finding beautiful pieces and I have to change a room to make them fit." Plus, he adds cheerfully: "The house is rather large. So we always have new places we can work on a little bit."