A backyard guest house may seem like a luxury few can afford ... On a cost-per-square-foot basis, building a small guest house is no cheaper than building an entirely new house.

they can be expensive to build. These backyard homes, called accessory dwelling units, or ADUs, are small housing units ranging from 150 square feet to 1,200 square feet, depending on where you live.

These homes, called accessory dwelling units, or ADUs, have their own address, separate from the main house they sit ... on end," she said. "By building an ADU in the backyard, the parents can ...