When the building of a mansion rivals that of a commercial project, the skills and talents of experienced home builders and architects are that much more valuable. Not since the Gilded Age/Progressive Era of the Robber Barons have so many millionaires been created by an economy, and subsequently built megahomes reflect their glory.

Microsoft mogul Bill Gates has built what has been described as the world's most expensive residence. The 40,000- square-foot mansion is situated on five acres on Lake Washington, outside of Seattle. Valued at close to $50 million, the home was in construction for more than seven years. Gates, the richest man in America, with a fortune estimated at about $38 billion, has built a unique home. Complete with a manmade trout stream, the high-tech home includes a number of electronic innovations, including heated marble flooring, a 60-foot pool, a sauna, a 1,700-square-foot guest house, a trampoline room, a 20-seat theater, an arcade, a 24-screen video wall, a sport court, a dock for water-skiing, two spas, and a reception hall for 100 people.

In Orlando, Fla., the home of Bettie Siegel, ex-wife of timeshare baron David Siegel, reportedly cost more than $40 million. Annual property taxes alone are $175,000. Known as Palazzo del Lago (Palace on the Lake), the Lake Butler home includes 13 bathrooms, six bedrooms, a boat dock, a swimming pool, and an elevator, as well as a 2,436-square-foot guest house. Lavish homes such as Siegels' are designed to offer spectacular views from all sides.

Examples of megaspending are available from coast to coast, as the new rich build their residential monuments. According to Clark French, broker/owner of Luxury Home Mortgage International, "Wealth is being created at the fastest rate in history, and the money has to go somewhere."

But what is driving the need for such massive, office building-sized homes? One architect offers this insight: "Many people building these homes entertain a large number of people on an international scale," says Bob Miller, owner of R. Miller Architects in Maitland. "In a way, they need a house this big. These people have corporate leaders over from Brazil, from France. They just have a bigger idea of what it is to have a house and to entertain."

These homes sometimes present building problems and have spawned a specialty within the new home community: builders and architects who specialize in megamansions. Among some of the concerns are load-bearing walls, foundations, and roofs, which can be many times the cost of a normal installation. If a roof is made of slate, it will require a massive roof structure, costing as much as 12 to 15 times that of an ordinary roof. Standard joists must be replaced by steel I-beams for living spaces over 14 feet. Insulation materials must be thicker to ensure adequate insulation in much-harder-to-heat large rooms. Luxury builders also wrap plumbing to deaden sounds, important in a home with double-digit bathrooms, multiple wet bars, and as many as two working kitchens.

One-of-a-kind amenities call for special engineering and architectural skills. Among many amenities favored by the megarich are such features as underground garages, cave-like swimming pools with waterfalls, movie theaters, commercial quality kitchens, elevators, and elaborate security systems.

"This is the same architecture used for museums and castles," says St. Louis home-builder Les Grotpeter during an interview in Orlando. "These beautiful houses bring back the 1920s with their millwork and stone." Grotpeter's clients' homes take as long as three years to build.

Cole Smith, the Dallas-based architect who recently completed a massive renovation on Dallas Cowboys owner Jerry Jones' home, says: "Some want a home for show, and others want an enclave where they can feel safe and retreat. When I was in Europe studying with the Parsons School of Design, it was pointed out to me that the glorious homes there always had an opening to the street. This was a way to enhance civic involvement and pride. Now, our mansions are not part of the city; they are hidden behind walls. So they really aren't a part of the fabric of the community. And that's too bad."

Philanthropists such as Jones often open their homes to causes, knowing that the curiosity of people who want to see inside will help the cause. During a recent fund-raiser for Southern Methodist University held at the Jones mansion, tables went for $15,000 apiece. For people who want to share their homes, making room for traffic, parking, tents, etc. is part of the overall planning, Smith says.

For all of these reasons, experienced architects warn against enlarging normal-sized house designs to scale to a mansion. The logistics involved are akin to overseeing a huge commercial project that has the finish-out of "artwork," according to one architect. The uniqueness of these homes and their owners is a prerequisite of the design, according to Smith.

He adds, "What it comes down to is how the owners want to use the home."