Landry says problems like this shouldn't happen with the proper engineering and construction methods. Done properly with the right shoring, he says, a deeper foundation can make the house more stable. 

But even Landry had to gamble with one underground build. To add space to a client's house, he was forced to lift the entire 5,000-square-foot house off its foundation by inches, while tunneling out new space and a new foundation over the course of the year. If one of Los Angeles' frequent earthquakes had hit during that time — at least one of any size — the whole project could have been in trouble.

Moreover, points out Scarisbrick, you reach a point where space no longer pays dividends. The farther you drill down, the more natural light you lose, making the space more pleasant for cars than humans. "I have seen developed houses where the space seems to be there just for the sake of it, creating an unbalanced house and no useful purpose for the space," Scarisbrick says.

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Developers, he says, usually cover up these deficits with "toys and gimmicks," he says, billing the dens as prime space for golf simulators, Pilates and meditation rooms. But it's not always convincing.

One of Kramer's Beverly Hills listings took a long time to sell, simply because the house above wasn't as grand as the extensive square footage below. "The scale above didn't match. What they really should have done is build a brand new house," he says.  

Also, brokers say, many buyers want their wealth to be on display above ground, rather than hidden below layers of pavement.

That listing in Irving, Texas, for instance, which looked so modest from the street, has lingered on the market, Brantley says, because many of the buyers wanting that much space want a more imposing facade.

"I think it has limited the potential pool of buyers," he says. "A lot of people who want a $2 million house, want it to look [from the street] like a $2 million house."