An ADU or 'Granny' unit can be an excellent way to augment family living, most often for teens or elders. Or as a rental, depending on the jurisdiction. Here is one designed for a family who wanted a space 'next door' for an older son. It has a 500 sf garage at ground level with a one bedroom living unit above.
A last note: context is important in any design. You need to at least know the context in order to 'fit' in as seen here with the ADU in the middle. Or to stand in contrast.