Neighbors and local officials have heard them all. But implicit in all these rumors about the outlandishly grand building emerging from the rural hills of the Ozarks is this nagging question: Why there?

The building, called Pensmore, promises to be one of the largest residential homes in the country when construction is completed — 72,000 square feet of living space that includes 13 bedrooms, 14 bathrooms and enough amenities to entertain a small city.

But instead of appearing in New York or Florida, where lavish living requires no explanation, the mansion was built in a thickly wooded plot of land in Christian County, halfway between Springfield and Branson. The nearest major landmark is a cathedral that bills itself as the smallest in world.

"It's really strange, and we're thinking something's up," said Michele Grider, who lives just down the road. "If there was a nuclear thing they could hide a bunch of important people there."

The owner, Steven T. Huff, 60, has heard all the "goofy speculation," as he calls it. His background — he sold the company he founded that makes software for military and intelligence agencies to Textron several years ago — has encouraged the rumors. But he said that his current work prompted him to build such a large second home.

As a major investor and board chairman of TF Concrete Forming Systems, he wanted to demonstrate the viability of new concrete technology that he believes will lower energy consumption. The size of the building, which is significantly larger than the White House, partly reflects a desire to build at a commercial scale for testing purposes, he said. He adds that he plans to have many guests.

The house — officially a single-family residence — has been under construction for nearly two years, and a recently posted Web site shows its progress. There are probably another two years to go before it is completed, which has irritated some neighbors who say the work has disrupted the serenity of the area and has cut off what used to be prime hunting grounds. Mr. Huff would not reveal the cost. (Though the purchase price of the land is not public, Christian County records show the more than 500 acres, historically used for logging and grazing cattle, was appraised at just over $50,000.)

Mr. Huff, a longtime resident of Virginia, chose the site on Woods Fork Road because it is just hours from his boyhood home. Also, he said building in that part of the country would help him show how the technology handles both hot and cold weather and even tornadoes. Finally, there is another advantage — by locating in an area famous for a small government approach, he is not subject to building regulations or inspection, which he said would have complicated his efforts.

"We try not to be more intrusive than we have to," said Todd Wiesehan, planning and zoning administrator for the county. He added that Mr. Huff had nevertheless invited some local officials to tour the construction site. "You don't appreciate just how large it is until you drive up," Mr. Wiesehan said. "It's very, very big."