This site could be titled Strata, Company and Community Title Living but a “Granny flat” is perhaps similar to a two unit strata without the formal by-laws etc.
Anyway I know someone who designed one and one of the biggest mistakes was putting plantation type shutters in the bathroom; both windows were in locations and of a height where privacy was an issue, they didn’t check the details of the shutters ordered – they presumed that the width of the shutters was the distance between the frames but there was a sub-frame also which meant the opening width was about 15cm less than they had planned. They also don’t let in much light unless fully open; so windows with frosting on the lower section which could be opened at the top only would have been a vastly superior and cheaper option.
Very careful consideration of exactly where they will be and what the priorities (light, air, privacy, security, place for a window box, insulation qualities, tinting) are is the way to go – including checking where summer and winter sun will enter. Plantation shutters were presumably designed to block out harsh light in tropical environments. One of the sadder sights in Australia is of houses in hot areas with windows facing West and no awning or significant eves to shade them in summer.