I live east of Lake WA and have never heard of your builders rationale. When I built houses a few years ago we put decks in level, unless there was a driving reason to do differently.

Ledger failure is a common cause for deck structure collapsing. It would be better to go above the existing ledger vs attaching to the face of it. Sounds like a 2x4 laying on top would bring you up about right. Not saying you couldn't attach to the face, but some more data would help. What is the size of the ledger? Also, if you attach to the face, extending the top of the addition up, you will be creating a channel to hold water and debris between the new board and the house wall. Not something I would want to do. In my opinion, moving the ledger up by placing a 2x4 above the existing would be the best, but not the easiest fix.

If you've got the size to work with, run your joists 3-4" high above the ledger. Maybe increase the size of your joists - if you've got a 2x8 ledger, run 2x10 joists. Push 'em up so decking will be where you want it and lock joists to ledger w/ joist hangers...

Mac

Here in MN the lower ledger board is virtually a requirement and considered good building practice. You could redo the ledger board and move it higher or increase the size of the joists to compensate.