“The house isn’t fitting as they expected it to,” according to architect Bruce Anderson, whose construction plans include new children’s rooms and a link from the main house to a 1,900-square-foot guest “pavilion.” (A Microsoft spokesman refuses to comment on rumors that another little Gates is in beta.) Construction costs are estimated at $750,000, but considering the home’s $113.5 million market value, Gates shouldn’t have to worry about a second mortgage.