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When is a house not a home? When it’s a $30 million to $50 million, 40,000-square-foot, seven-pavilion waterfront estate jokingly referred to as Bill Gates Interplanetary Headquarters.

What may be the world’s most closely watched abode is coming along nicely. It’s not quite on schedule, though. News stories in 1990 predicted it would take at least two years to build.

But it’s an impressive piece of construction nonetheless. Projected date of completion is late 1996, more than a year later than planned.

Part of the delay may have been the result of Gates’ marriage to Microsoft executive Melinda French on New Year’s Day, 1994.

After looking over the plans and reportedly saying, “I’m not living in that!” French became intensely involved in the project. She is now said to be pleased with the progress.

The Medina estate, built into a bluff above the shore of Lake Washington, will include:

– A 12-foot-wide “video wall” in the reception pavilion composed of a series of big-screen TV monitors to show digitized artwork.

– An L-shape swimming pool with music piped in underwater.

– A 20-vehicle concrete garage.

– A 9.5-by-16.5-foot trampoline pit. Gates, who can jump over an armchair from a standing start, has always loved trampolines.

– A theater seating several dozen people and a dining room for several hundred guests.

Contrary to persistent myth, the house is not as much as 80 percent underground. Its foundation and substructure is built into the hillside above Lake Washington, but living quarters are above ground.

Gates has said that the house will be a window into 21st Century living.

But visitors who have toured the site say most of its technology, while big and expensive, is typical of what can be found in state-of-the-art mansions.

Some of the technology the Microsoft mogul originally envisioned-for example, flat-panel displays, painting-sized versions of screens commonly found on portable computers-is not yet available; provisions for adding them are being put in place.