Some rich folks building their dream castles “are buying a stone quarry, harvesting the stone, and shutting it down, so that no one else can have it again,” Murphy said. “It’s happened multiple times.”

Details are crucial: It’s not enough for a wall to be fine marble, it has to be book matched, so the lines flow naturally. “It’s expensive and time consuming,” Murphy said. Onyx and mother of pearl are in vogue.

The bigger the better. One custom house Coastal is building in Hillsboro Beach is set to be 60,000 square feet. That’s about the size of 50 two-bedroom apartments. “A beautiful big home,” Murphy said.

A huge issue: South Florida — especially Miami — is running out of prime oceanfront and bayfront sites. “Land has been scarce. A majority of the projects we’re touching are all knock-downs. Or it’s been knocked down by the time we get there,” Murphy said. “You’re seeing more people buying not one lot, but one, two, three.”

Coldwell Banker luxury super-broker Jill Eber — who works with Jill Hertzberg as The Jills in marketing many of South Florida’s most exclusive homes — said tear-downs are increasingly common.

“We just closed at the end of the year on a property at the tip of Sunset Island for $20 million,” said Eber, whose team had the listing for the record $47 million Indian Creek sale and closed on some $480 million in residential deals in 2012. “Somebody bought it for $20 million and is tearing it down. It’s only about 46,000 square feet [of lot], but it’s got 200 feet on the open bay looking directly downtown.”

Another likely tear-down, added Hertzberg, is 15 Star Island, which is listed for $16.9 million.

“At the highest end, a luxury home is of the moment, right now,’’ Hertzberg said. “It’s current, with all the bells and whistles and amenities.’’

Many buyers at the top of the Miami market are snubbing the long-popular Mediterranean-style architecture of barrel-tiled roofs and stucco walls in favor of sleek, contemporary, mostly glass houses.

“People are asking more and more for contemporary, clean lines,” said Audrey Ross, senior vice president of Esslinger-Wooten-Maxwell and a top-selling luxury Realtor who has sold many of Miami’s prime estates several times.

Wayne M. Boich Jr., the 37-year-old CEO of the Boich Group of Companies, a Columbus, Ohio-based coal business, bought three adjoining bayfront properties at 4666, 4682 and 4700 North Bay Rd. in Miami Beach for $17.7 million last July and knocked down the homes to make way for a sleek, contemporary home bigger than all three.

Boich, known on the South Beach party circuit for throwing lavish gatherings, sold his penthouse at Icon South Beach for $21 million last year.

At North Bay Road, he and fiancée Cynthia Marin, who plan to marry in March, are building a 17,975-square-foot crystalline glass palace designed in a series of horizontal rectangles perched on a wide swath of Biscayne Bay.

“Finding the right lot took a significant amount of time. We were really happy to gather this together. It’s been really exciting,” said Boich, who hopes to move in early next year.

“It’s a modern feel, a sleek feel.”

Miami architect Kobi Karp, who also is doing some of Miami’s most upscale condominiums, said effort was focused on taking advantage of the spectacular view of Miami and downtown. The building, which he describes as “floating rectangles” will have “huge verandas and terraces to create shade” on the mostly glass façade, he said.