Guys, take it from somebody who spends their time preparing planning applications and dealing with Pre-Planning Consultations....The Planners do not tell or help you get to what you want. You have to show them what you are planning to do and take their opinions on board while demonstrating that the extension in in line with local development policies.
To be honest, most domestic extensions don't go for Pre-Planning, it would typically be one off dwellings or bigger unless you are in an ACA or a Protected Structure.
The dormer issue can be answered by looking for precedent in your immediate area and your Planning Agent should be doing their homework on this.