A home that is over six thousand square feet is called a mansion. Mansions are usually built with very elaborate and strong materials. Sometimes they are even built with strong brick walls surrounding the perimeter to keep out paparazzi or lunatics. These buildings can become a very easily defended area. In addition, mansions are built with many rooms and can support a large group of survivors. Also, if a mansion has enough land around it, that land can be converted into farmland and can support the survivors living there.
Mansions have several stories that can make it easily defendable with snipers and other long-range weaponry.
A mercenary interviewed in World War Z talks about a wealthy man who hired him and several other mercenaries to defend his mansion (probably in Long Island) when The Great Panic arose. This client of theirs had offered a safe shelter for many celebrities, mainly to boost his own ego. They were well supplied with plenty of fuel, food, and ammunition, and the mansion itself was essentially a fort. Unfortunately, the celebrities were foolish enough to stream their exploits live on the internet and several hundred people flocked to the mansion, hoping to get to safety. As they invaded the mansion, engaging in a tragic shootout with the armed celebrities and their employees, the attacking commoners presumably damaged the mansions outer defenses so severely that the luxurious base fell almost immediately in the face of an undead attack.
In 28 Days Later a squadron of British Soldiers took shelter in an abandoned estate or mansion, known as Worsley House, and used it as their base against the Infected. The soldiers fortified the mansion with a perimeter fence, floodlights, land mines, and trip wires.
During World War II many European estates were taken over by Allied Military and were used as front-line positions for soldiers to defend.