Little-known facts about Bill Gates' $124 million mansion

The guest house is just as high-tech as the main house.

According to US News, the 1,900-square-foot guest house was the first building to be completed on the property. The house — which has its own bedroom and bathroom — was meant to be a test of the technology that would eventually be used in the main house.

Gates wrote much of "The Road Ahead," his 1995 nonfiction book on the future of tech, at this guest house.