Set on a 7-acre vineyard in California’s Napa Valley, a compound known as Yountvilla is a private second home designed for entertaining a large family.

In addition to the 14,000-square-foot main residence—in what Oakland, California-based architect Toby Long calls Napa-barn style—the estate includes a 2,000-square-foot pool house and a 2,000-square-foot party barn. The cinema, conservatory-style great room, swimming pool, hot tub, outdoor kitchen with two pizza ovens, large reflecting pool, six-car garage, tennis court and two outdoor terraces bring the party home. But for all its singularity, the lavish estate is among a growing number of modern modular mansions springing up across the U.S. that feature prefab factory-built components.

Ultra-high-net-worth individuals, some driven by the need to sequester safely during the pandemic era, have chosen to erect these houses, which can cost millions and even tens of millions of dollars, because they are more efficient to build, are of superior quality, and most significantly, they can be completed far more quickly than those built via traditional on-site construction methods.

Mr. Long, who has been building prefab houses for over two decades under the brand name Clever Homes, said that the genre “is emerging from its slumber in the U.S. When you mention prefab or modular, people think of high volume, low quality. But it’s overcoming its legacy of cheapness—it’s a sophisticated process.”

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