Granny flat, backyard home, carriage house, in-law unit, guest house, these are all different terms for an ADU. ADUs can be attached or detached from the main house and are a separate living unit complete with its own entrance, living, kitchen and sleeping areas.
Given the under supply and high cost of housing in many cities throughout California, the state recently enacted legislation that promotes the construction of ADUs and requires cities to make it easier for homeowners to build ADUs on their properties.
Detached ADUs stand on their own, independent of the main building. Typically, they're smaller structures built in the backyard away from the house or garage. In different areas of the country, people call detached ADUs granny flats, backyard cottages, or laneway houses.