many of the models in bonita canyon have this feature, sometimes with the option of the "bridge" connecting to the main house you speak of. they do often have small kitchenettes i think (or at least a sink and fridge) as well as full bath. i know several who do use the units as a guesthouse. this way the guests are free to come in and out as they please without access to the main house. i've stayed as a guest in these homes and i actually enjoyed the privacy and i was more comfortable knowing i wasn't disturbing my friends much. they still had their jobs to go to and own schedules to take care of, and it allowed me to come and go as i needed during the day. and they didn't need to leave me their keys or worry about me going in and out of their home. it's comforting to both parties. i would never want to let one of my kids live there although i'm sure every child would want that room as soon as they hit puberty.
technically, without a full bath and kitchen, it would not be a legal rental. also in practical terms, they would need access to a washer/dryer which these days is on the second floor of your main house. i'm sure people would still rent them out anyway.
http://www.dca.ca.gov/publications/landlordbook/problems.shtml