Frock is still used in the U.S. mostly in the context of clothing advertisements and usually refers to a somewhat fancy waistcoat made of woven fabric (in contrast to a knitted sweater type of affair).
bjd, in the States we'd call your "granny flat" a "mother-in-law" which refers to a separate, fully self-contained residence located within the main house's property. This usually refers to a separate unit, but can include such things as a room over the garage, a basement abode, or any housing in which you could stuff your mother-in-law separate from your living space.
