When Joseph Pell Lombardi first saw the Armour-Stiner Octagon House, the only fully domed eight-sided residence in America, it was, he says, "a deteriorated treasure calling out for help. You could see the shape, but everything else was disguised."

Joseph Pell Lombardi stands in the entry hall of the restored octagon house, where the walls feature a baroque trompe l?oeil stenciling in silver-colored aluminum. The furnishings are Renaissance revival. The house is the only fully domed, eight-sided residence in America.

Lombardi, now 66, a professional preservation architect, bought the 19th-century home in Irvington in 1978 for $75,000, and has been bringing it back to its halcyon days ever since. The process has involved more than 100 people, including researchers with graduate degrees in historic preservation and workmen with archaic skills like trompe l'oeil painting, gilding, fine carpentry, and glazing—all paid for from Lombardi's own pocket. (He refuses to discuss how much he's spent.)

Today, the building, designated a National Historic Landmark, is a visual knockout: a Victorian fantasy of a Roman temple, or Haight-Ashbury meets the U.S. Capitol building. You can see why just by looking at the exterior: an eight-sided, wraparound verandah, its roof supported by 56 columns and surmounted by a monumental slate dome, an observatory, and a spire. Decorative carvings garland the entire structure, and cresting rises toward the sky. Colors include surprisingly harmonious shades of crimson, dusty pink, charcoal, dove gray, terra-cotta, violet, lavender, and tan.

The interior of the home—some 20 rooms spread around a central staircase over five floors—is almost as ornate. The walls are painted and glazed lyrical colors like ocher, aqua, sedge green, and baby blue. The ceilings bloom with multi-colored moldings and medallions so shapely they border on sculpture. Tying all the elements together are the original floors, in woods like walnut and yellow longleaf pine; etched glass doors; windows onto the verandah; and florid Renaissance Revival furniture contemporaneous with the house.

The ornate verandah railing surrounds the house in color.

There were few hints of such splendor when Lombardi purchased the house from the National Trust for Historic Preservation. The slate and much of the carving and cresting were gone. The outside paint was a peeling gray and white, the inside off-white. All the original furniture had disappeared. Worst of all, the dome was caving in, splitting apart walls in the upper floors. "You could see daylight between the walls in the third-floor music room," Lombardi says. "Birds were flying in and out."

It was not even clear if the dome could be fixed, but Lombardi bought the house anyway. "When you have a passion, you take risks," he says.

Lombardi's passion for saving old buildings was already well established. He and the New York City firm that bears his name had been restoring downtrodden Manhattan townhouses since 1969, and were pioneering the conversion of derelict commercial structures into historically correct but modern residences in the city's Soho and Tribeca districts. By the 1970s, he had also begun buying and restoring antique homes for his personal study and use. (Today, he owns a total of five.)