The cinder-block house, completed in 1959, was built on a steep hillside overlooking the Potomac River in McLean, Va. Its 2,500 square feet of living space are contained in a structure that appears solid from the front but on the inside reveals its spectacular view through 80 feet of floor-to-ceiling windows. By the time the Mardens reached old age (Luis died in 2003, and Ethel, now in her late 90s, lives in an assisted-care facility), the house was badly in need of repair. And, given its desirable site, it was a sitting duck for someone who wanted to tear it down.

But an unlikely white knight happened to live next door. James V. Kimsey, a philanthropist and the founding chairman of America Online, had recently completed an enormous and expensive house for himself — he refers to it wryly as “a monument to wretched excess” — when he bought the Mardens’ house in 2000. He had no interest in living in the diminutive structure but saw its potential as a guest house and a place for parties. “I thought of making alterations,” he recalled, “but people told me not to. It wouldn’t be a Frank Lloyd Wright house anymore, they said.”