Apart from all the additional advantages of Earth Sheltered Buildings referred to on the other pages of this website - Earth sheltered Granny Flats have their own specific benefits.
LANDSCAPED AREA. On many sites an earth sheltered building can maintain much of the usable landscaped area so it might be possible to keep most of your backyard and still add in a substantial Granny flat. This can be an advantage in getting building approval because many councils have a Landscape Ratio which might otherwise restrict the size of an additional dwelling.
ACOUSTICALLY INSULATED FROM REST OF HOUSE. An earth sheltered granny flat can be more easily acoustically isolated from the main house because of the volume of the surrounding earth, which can be beneficial if you are planning it for noisy teenagers or to rent out to tenants who might want a lot of privacy.
BUSHFIRE RESISTANT. We believe we can design a Granny Flat which in all likelihood could not be damaged in even the worst bush fire. See Bushfire Resistant Houses