Condon's clients have requested 1950s-style diners and re-creations of their favorite pubs. Another trend, he says, are "Zen areas," relaxation spaces and yoga rooms—often boasting water features—that connect to full gyms, which sometimes even have locker rooms.

Joe Farrell, owner of Bridgehampton, New York-based custom homebuilder Farrell Building Co., says a standard basement in the resort area comes with elevator access, a full gym, a movie theater with stadium seating and concession stand, a wine cellar, and a sauna and steam room. He's currently building a house with a basement featuring twin bowling alleys, a kids' recording studio and performance space, and a "princess area" with a miniature beauty parlor.

"Basements used to be the thing where somebody would want to spend 15 or 20 grand and gussy up a little finished area," says David Crane, principal of Crane Builders in Nashville. "Now people want basements as nice as the rest of their house."

Bob Hampton, the former president of Jones Media Group, which was sold to Triton Media Networks in June, has four living areas in his Denver home, including one in the master suite. That didn't stop him and his wife, Janis, empty nesters, from finishing about 2,200 square feet of their basement. It now has a bar, wine cellar, steam room, media room, and finished storage area. The initial plan was to create a home gym—with a television for each of them—and a ballet area. But, says Hampton, who spent about $300,000, not including furnishings, "We hated the thought of just finishing a part of it."

That type of evolution is increasingly common, says Philadelphia-based designer Mark Oser, noting that dedicated media rooms ignited the trend. "Now that you're doing this high-end theater, you can't just put it in an ordinary basement," he says, deconstructing the new basement psyche. "That leads to, 'Well, it'll be nice to have a bar with it, and a living room with a fireplace.' 'Well, we don't feel like running upstairs; let's put a bathroom down there, let's put a whole kitchen down there.'"

And everyone from builders to brokers to homeowners are cashing in on the trend. "I try not to use the term basement," says DePersia. "I call it the concourse level."