In the 19th century, families like the Astors and the Vanderbilts spent years or even decades designing estates to impress European aristocrats. Now it’s developers like Mr. Niami, a former B-movie producer, who are building the homes, designed to impress international billionaire would-be buyers.

The house’s architect, Paul McClean, who also designed the home that Jay-Z and Beyoncé paid $88 million for earlier this year, said it would be as much an entertainment showpiece as a house. That’s very much how Gilded Age mansions functioned. “The pattern repeats itself,” Mr. McClean said.

But while Gilded Age mansions were built as family legacies to be passed down to future generations or endowed to universities, these tech-centric, ultramodern glass-and-marble behemoths are designed for living in the moment. They come furnished, often with artwork, wine and cars. Each represents a developer’s bet that an instant-gratification billionaire is willing to pay more for it than almost anyone else has ever paid for a personal residence before.

This New Gilded Age has found an epicenter in Los Angeles, particularly where Bel-Air, Beverly Hills and Holmby Hills converge. Real estate agents call it the Platinum Triangle. A spec home is on the market in Bel-Air for $250 million; it comes with two years of prepaid household staff. Nearby, a London-based developer is marketing a gated community where homes start at $115 million.

In 2012, Mr. Niami paid $28 million for the hilltop lot, which included a vacant 10,000-square-foot house that he said was in ramshackle condition. He declined to say what he was spending on construction.

The One, as he has branded it, will officially hit the market when it is completed in mid-2018. Local real estate agents seem to agree that Mr. Niami is building a one-of-a-kind mansion on a one-of-a-kind lot, with a 360-degree view you’d be hard pressed to find anywhere else other than the Getty Museum.