Just seen that GF's can now be 90m2 in ACT, and don't need to be related to occupants:
http://www.abc.net.au/news/2014-02-10/larger-option-for-new-act-granny-flats/5249698?section=act
http://www.abc.net.au/news/2014-02-10/larger-option-for-new-act-granny-flats/5249698?section=act