Basement conversion ADUs are also referred to as “basement apartments” or “English basements”. This is a convenient option provided you have a basement, it’s not being used, and it’s large enough for a bed/bath/kitchen. Most homes in California unfortunately don’t have basements, which would likely make this project more trouble than it’s worth.
In some cases, basement ADUs such as the one shown below have their own entrances from the outside:
However, this is not the norm. Most basement conversions can only be entered by going through the primary home. This may not be ideal if you are planning to rent it out to strangers or are trying to maximize privacy for both the inhabitants of the main house and the ADU.