Luxury real estate developers are building out elaborate basements for multimillion-dollar mansions, and they include spas, tennis courts, and even ballrooms

Developers are taking luxury homes in an unexpected direction: Underground.

Rochelle Maize, a Los Angeles luxury real estate agent, said she's seeing more and more "extravagant basements" or lower levels being built into homes.

"While this square footage isn't considered as valuable as above grade, it still allows for a higher selling price and return on investment," Maize told Business Insider.

But these spaces are nothing like the dingy, unfinished space you might picture when you think of a typical basement.

"The other big trend that I am seeing in the basement level build-outs is interesting details that are being installed by landscape architects," Maize said. "They are creating gorgeous 'living-walls' on the concrete wall areas below ground. This is actually making them more inviting than some of the above-ground areas!"

Some of these lavish subterranean spaces include spas, wine cellars, art galleries, tennis courts, and other sports facilities.

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In Los Angeles, these spaces are often a way for developers to get around "red tape" that restricts the size of new construction single family homes, known as anti-mansionization laws, Maize said.

Hacienda de la Paz, a 51,000-square-foot home in Los Angeles county, was built to be mostly underground in order to comply with such zoning requirements, according to the Los Angeles Times. Only one of its floors is above ground, and the other five levels are underground and include a 10,000-square-foot Turkish spa, a 15,000-square-foot tennis court, a ballroom, a yoga room, and a wine cellar.

It sold at auction in October for $22.4 million, the Times reported.

Then there's the most expensive home for sale in San Francisco, an ultra-modern complex listed for $45 million, which has a two-story underground art gallery, as Business Insider's Lina Batarags previously reported.

These over-the-top basements are not just a West Coast trend. in the wealthy Hamptons community of New York, told the New York Times about a house he built with a 5,000-square-foot underground space that included a bedroom, a wine cellar, a home theater, a gym, a sauna, and a steam room.

"You don’t have a feeling like you're underground," Collé told the Times. "These are open, airy spaces."

The super-wealthy of London have also been building downwards, Business Insider previously reported, sometimes adding up to five levels underground in some of the city's most sought-after neighborhoods.

Luxury real estate developers are building out elaborate basements for multimillion-dollar mansions, and they include spas, tennis courts, and even ballrooms

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