This isn't the biggest flat I've lived in as an adult, but it's far from the smallest. Personally, the back garden thing throws me a bit. You feel like it's your own personal space, but then suddenly there's someone standing a foot or so away from you in their personal space. I don't get the etiquette.
On one side, my neighbours ignore us when we're out. I am not sure if I should try to not go outside when they are out there, but they seem to avoid it when I am out there. They also usually shut their back door if they notice me out. On the other side we have our upstairs neighbour. He will strike up conversations, sometimes through his back door. It will occassionally scare me spitless if I am out gardening and in my "zone". Our garden is sort of "L" shaped, so we have a third bordering garden, which is perhaps the freakiest of all. We're a garden apartment and we're slightly below the grade. As you leave our back door, we have a deck area and climb some steps into the back garden. The people in the garden on the other side of our upstairs neighbour can see straight into our bedroom. I didn't realise this until I was napping and the guy in that flat was cutting his grass. I looked up through the window to see him towering over the fence that borders his and our upstairs neighbour. For some reason, it's almost like his garden is above even our grade. Of course, we can't put up anything to block the view, at least on the outside, as this will probably block some sun and air.
But don't get me wrong, I love that we have a garden. I use mine a lot, probably more than my neighbours combined, and I sometimes wonder if I make it so others feel they would be intruding if they use theirs. It just seems so weird to have such a massive garden and yet because of how the housing is set up, feel we're right on top of each other.