Hi RN,
I am thinking the same as you are, and would like to build into my block's rear set back. I know the on my block I can encroach 500mm into the front setback for the floorplan of the home or 600mm if it is eave. There is no mention of any options for the rear setback. Although if you speak to Austaland they can give you some advice on the conditions of the estate and in particular for your block. The Council will review whatever you put forward, however you need to make sure that firstly Autraland will not object to and then also there is the Council and the estate residence that have vito over any change you what to make to the building envelope. I am about to take that step myself.
Deposit land 14th April 2012
Settlement land 22nd June 2012
Jason