Depending on the council having one larger room and a smaller study may save you on Section 94 contributions as some are charged according to the number of bedrooms being added. Nothing is stopping someone from using it as a bedroom for a child.
That is a VERY good point western_sydney. Of the councils that are still charging Section 94 contributions for granny flats, some charge on the basis of the cost of the development whilst others charge on the basis of number of bedrooms.