Mr. Gates, who co-founded Microsoft in 1975, began assembling his residential compound in the late 1980s, buying up various homes and parcels in an apparent effort to maintain privacy. He purchased the house that is now likely under contract for $1 million in 1995, about a year after marrying Melinda French Gates. Over the years, several renovations took place there, according to property records, including the addition of an expansive deck.