I searched for local streets on there and also wandered around the neighbourhood looking for dormer windows in houses that looked nice to give some others to search.
I've also dug through all my neighbours plans on the local planning portal. Good place to start in terms of seeing what's possible given the space and layout of similar houses.
Your local council should have some kind of online planning portal (this is Haringey's for instance http://www.planningservices.haringey.gov.uk/portal/servlets/ApplicationSearchServlet ).
I searched for local streets on there and also wandered around the neighbourhood looking for dormer windows in houses that looked nice to give some others to search.