Sustainable development dissertation title?

You don't need the exact question yet. You've mentioned the area you'd like to cover, so look into it. There will be plenty of papers published on how planners can contribute to SD so do some research into how they do so and then think about the pros and cons of it from all points of view (ie use critical objectivity, which is pretty much the whole point of uni work). By then you should be asking yourself questions about what the planners do, so all you have to do is pick one of those questions and make it your dissertation question.