In my town you need a permit just to redo the roof surfacing, but no drawings. I would be shocked if your town didn't require a permit for a new dormer, but as for drawings, you'd need to check with your town. Drawing requirements vary greatly. I've had projects that had to go before historic commissions, and needed a full set of plans, and other projects that merely needed to show the calculations and a basic hand drawn framing plan, for the same type of work. It really is up to the local jurisdiction. Best bet is to ask.