60m granny flat on top of 60m outbuilding garage possible in NSW Hornsby Shire?

Hi,
Is it possible to build a 60m^2 granny flat as the second story on top of a 60m^2 garage where both are separate from the main dwelling?
The maximum granny flat size is 60m^2 in NSW.
In Hornsby Shire where I live the maximum outbuilding size is also 60m^2.
I would like to build the biggest separate dwelling possible and think that this might be the best way - build an external garage and a granny flat on top. Is this allowed? So far as I can see it is, judging from the Hornsby Shire Council Development Control Plan (DCP). See page 7 here for the definition of an 'outbuilding':
http://www.hornsby.nsw.gov.au/__data/assets/pdf_file/0008/77048/HDCP-Part-3-Residential-v2.pdf

Background: My property is 800m^2 zoned as R2. This is too little to subdivide in Hornsby Shire where the minimum block size is 500m^2 for this zoning. The only existing dwelling is 95m^2 single story. I have a flat grassed yard that I'd like to build something cheap on and rent out.

Thanks for your advice.
Cheers,
Keith

 

I think the overall height would cause issues.
Setback penalty is quite harsh. Where you exceed the height of 3.8m, your setback needs increase by 3x the excess.

 

For the setbacks etc that @neK is referring to, see here: NSW Legislation

 

Hi Nek and Propertunity,
Thanks for the suggestions and links, that's interesting about the setbacks.
Best regards,
Keith