free standing duplex behind one another possible?

Hey guys

I've been a long time reader of the forum. and I do love this place..
sad to say, I haven't contributed much.

I do have a quick question though, specially for those who may be familiar with the Bankstown LGA, other LGA might be similar in ways, so feel free to contribute..

So I am currently looking at a property in bankstown LGA approx ~800-900m2 15m wide.
As far as I am concerned this is fine for an attached duplex build, but would it be possible, to build free standing duplexes behind one another?

Another option I am considering, is keeping the existing brick home, and building a granny flat. Anyone know if there is a maximum allowable size for a granny flat?