The Rise of the Trophy Basement (The Wall Street Journal)

Wealthy home builders around the globe are digging deep.

In Los Angeles, builder Mauricio Oberfeld has buried about a third of his home underground: He built a contemporary 9,000-square-foot house for his family with a 3,000-square-foot basement. Glass stairwells lead to a lower level with an ornately tiled spa, large office, wine room and movie theater.

"What you see from the street looks pretty low-key," Mr. Oberfeld says of the home, completed in 2010. "I don't like to be ostentatious and showing the world."

Formula 1 racing heiress Tamara Ecclestone is adding two underground stories totaling 5,000 square feet to her 17,000-square-foot home in London's Kensington neighborhood, purchased last year for $70 million. Features include a pool bar, billiard room, bowling alley and a nightclub, as well as a large gym and a game room for Ms. Ecclestone's boyfriend. Designer Gavin Brodin says the basement will also include a 3-D movie theater with eight custom-designed seats, each with its own built-in refrigerator and popcorn maker. Ms. Ecclestone, who declines to discuss the project's cost, says she wants the space for entertaining and adds that she decided to build downward because the home's historic status prohibited aboveground expansion.

"Everyone's digging these days," she says.