Pitched roofs are preferable for snowy areas to prevent collapsing but flat roofs are great for rooftop gardens. Though I voted neither it really should be either or both because so much depends on the area and the need.

Just looking at your plans again, any chance of doing a pop up roof both sides? Do you have to reinforce all the walls in your downstairs area - starting from ground level? I was just thinking in for a sheep as a lamb, if you're spending that much money can you possibly spend a little more and get maximum use from upstairs. It's so worth it, especially if you end up with a view of something/anything. Something else to bare in mind in insulation between upstairs and down which will stop noise travelling. I don't think you will need insulation in your upstairs ceiling as it tends to get very hot up there! We have a butanol roof on top which is supposed to last at least 20 years. One side of our upstairs are small verandahs which are lovely but rather a waste of space, but if you have any sort of view they would be incredible and increase the value of your property immensely - not to mention they are just lovely in the summer.

Many solar installer recommend the degrees of latitude for you area as a starting point. In mid summer sun is 15° above the latitude, mid winter 15° below. Adelaide is 37°, so ballpark 22° for the mid winter lowest pitch. Good idea to speak to a few reputable suppliers/installers as well so as to determine which way they'll face, shading from tree's, etc. They used to say Northern side now they're saying West..... Northern or Western doesn't work at our place due to shading from tree's. So we will have to face them on to the North Eastern roof......

Yes, you need to check the planning regulations with your local council. Yes, your neighbour has built to the boundary near the front of the block, but it doesn't affect the light coming into your side facing windows as your house at this point is along way from the boundary. On the other hand your proposed extension is very close to your neighbour's windows and will reduce the amount of light that reaches them. Get your setbacks right before you get your roof designed - you may end up with a lot less roof than you had planned for! Best of luck, Dr Retro of Dr Retro House Calls and Dr Retro Virtual Visits