Yes, you need to be really careful, people are very educated and informed about whats going on in this area. Unless by some reason you have planning, i wouldn't go ahead. The planning authorities will be informed by someone.
From the look of the other thread Veronica has posted on the subject they either have or are in the process of getting planning permission.
Cool, I just know someone had a bad experience with her terrace after she went ahead with out planning. Just wanted you not to make a similar mistake. Best of luck, there are roof terraces on the ladder remember.
The key point for planning is, you are not allowed to sit on a roof terrace where your neighbours can see you observing them. Just put up a screen and you should be fine but it's VERY intimidating being in your garden and aware that someone is on their terrace potentially watching you. Funnily enough it doesn't raise the same hackles if they're inside their house.