I've heard them called "in-laws" or sometimes "granny flats." Ours was always, affectionately, the "mom shed," and we kept it entirely within the family: a new home for my mother, located in my sister's backyard and built by my brother-in-law. I only barely knew what I was doing at the time. I bought my brother-in-law a copy of Michael Pollan's A Place of My Own, and he bought me a tool belt. Both remain untouched to this day. We learned a few things, though, and we'll be doing more of these.
The photos below were taken three years after construction completed.