Double storey unit in the rear

Hi experts,

I am starting my first invest property, and now I saw a property which could keep the existing house and subdivide the backyard. The site is in Banyule Victoria, around 670 sqm, and the existing house sit is the half front. All looks good but the problem arise.

The site is GRZ2 with 40% coverage(270 sqm total ground building area). The existing house already took 150+ sqm, and requires a double car port with one under cover, so another 20 sqm gone, only left 100 sqm for the rear. So have to have a double storey to make this deal profitable.

I sent the site info to one town planner, he told me the chance of the double storey in the rear is less. I went to council in person, and asked the same question, the council told me double storey is allowed, the height is up to 11m.

So my question is how likely to build a double storey in rear after council said yes. Is it possible council reject the double storage plan afterwards for some other reasons?

Thanks
Ron