When a piece of property for a home is first purchased, the size of the family who will inhabit it often determines the size of the land they choose. A couple just starting out might select a small plot with a cottage on it, but they will eventually need more space as they increase their family. If there is enough room on their land, they might build an addition onto their existing home. It will be the least expensive way of adding living space to a property, but it is not the only way to increase it.
Building additional structures on a lot has been a traditional way for people to expand their living space, but there are other reasons for adding new structures. Some homes come without attached garages, and the owners might find their vehicles need a space of their own. Hobbyists who retire might suddenly find a need to move their hobby out of the house and into its own space, or they might decide to start a small business that requires more room.