How to attach ledger board to under-roof siding?
Since the new rafter section will cover any siding you may remove, layout how far the rafter section covers the siding and cut it about a 1/4 to a 1/2" lower. This will let you find the framing pretty easily, even remove a little subsiding to confirm, careful how deep you set your saw. This will show any conditions you may need to attend to. Set the bottom of the ledger at the on a chalk line set at the level you predetermined before you cut the siding. Set a ledger at the top of the rafter section as well to control rotation of the new section. This is an expansion on what Comintern suggested with his ledger and joist hanger idea, make sure the joist hangers approved for your framing condition. 

Here is a little sketch to help illustrate. Doing it this way will allow a finished ceiling to go uninterrupted to the existing wall.