WE CALL IT THE URBAN family homestead,” Cate Dunning says of the house she now shares with her husband, Chris, and daughter, Mollie. “Whenever there’s a holiday, or any event, everyone is in our house.” That tradition dates back to long before the home belonged to Dunning, an interior designer by trade and cofounder of Atlanta design firm GordonDunning. The story begins in 1955, when her maternal grandparents moved from Maine to Decatur, Georgia, and settled into “773,” a loving nod to the “homestead’s” address. They raised four children in the house, and, later, it was a hub for extended family. “We grew up going there all the time,” says…