Adding a dormer affects the external form of the building. This approach is more likely to require planning permission if your house is in a conservation area. Speak to an architect about your budget and requirements to establish if a dormer approach is suitable. As a minimum, loft conversions...
No extension beyond the plane of the existing roof slope of the principal elevation that fronts the highway (i.e. no dormer to face the street). No extension to be higher than the highest part of the roof (i.e. dormer does not increase the roof height). ...