Having rendered unknown, shady (and almost certainly magical) services to King William I, Malfoy was given a prime piece of land in Wiltshire, seized from local landowners ...
Armand Malfoy accompanies William the Conqueror from France to Britain with the Norman Conquest in 1066. He provides assistance — possibly magical in nature — to William and is rewarded with land in Wiltshire. The Malfoy family will maintain a stately residence on that land for the next 900 years.
The Norman Conquest was in 1066. However, King William spent much of the next decade consolidating power and putting down rebellions. He was finally secure in his throne in 1072. It is unclear when Malfoy was given the land in Wiltshire, but the assumption could be made that it would have closer to 1072 than 1066.