The unfinished "shells" of two penthouses in a San Francisco high-rise are on the market for $7.5 million and $10 million.

The two units were originally meant to be conjoined as a $49 million penthouse, which would have been the city's biggest-ever condo sale.

But the developer decided they might fare better if split into two customizable half-floor, two-story homes.

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The top floor of Lumina's Folsom Street Tower originally went to market in 2015 as a two-story, 15,000 square-foot $49 million penthouse. It would have been San Francisco's biggest condo sale in history if it had sold.

But it didn't. Eventually, "it became clearer that the market may be more receptive to two customizable homes," Tishman Speyer's senior managing director Carl Shannon told Business Insider in an email. So the $49 million penthouse was split into two units in 2017. And as of this month, they're listed on the market for sale.

Penthouse 41A is priced at $10 million and is 5,700 square feet in size and penthouse 41B is a tad smaller, spanning 5,200 square feet, with a $7.5 million price tag. They take up the 41st and the 42nd floors of the tower between the two.

They're being sold as customizable "shells," meaning there's no floor plan, walls, or interior design — which means that the future owners will basically be paying for unfinished apartments.

"We believe that buyers interested in these spectacular spaces would prefer to customize their homes to their personal specifications to better match their specific tastes," Shannon said. He said there are plans and interior layouts available for the future buyers to choose from.

There was once a $1.55 million build-out incentive attached to each of the penthouses, but they've since been removed. According to SF Gate, build-out costs for either of the units could be north of $5 million. So the build-out costs will come out of the buyers' pockets, meaning the prices involved to turn either into a living space could likely start at $12.5 million and $15 million respectively.

The Lumina offerings are just some of the luxury condo options in San Francisco, specifically in the "East Cut" neighborhood near the Financial District and the offices of tech giants like Salesforce, Facebook, and Google. There's a market for this kind of high-priced living thanks to the city's population of high-earning professionals, many of which are in the tech sector.

Take a look inside the more expensive penthouse of the two.