Planning stage of a deck...

I have an exterior French door with a 2x12 board serving as a foot plate below the sill. This plate is nailed to the rim joist.

I'd like to attach the new deck's ledger board to it (and the rim joist) with lag screws (and washer spacers between them).

By the way, the deck will be wider than the 12' board.

Is this a good approach?

Your idea sounds OK. Why do you want the washer spacing? Lag screw or nail the new one right up against the existing one to make a double plate, maybe use a piece of flashing across it's top to keep water from running in between them. Make sure everything slopes a little away from the house so rainwater doesn't flow towards the door or foundation. If you space this plate with washers all the weight is hanging on those little exposed areas of screw which could shear off instead of being transferred into the wood fibers.

Thanks -- re: washers -- much of the advice I've heard says that good drainage beats good flashing any day; therefore, the washers allow water to just run down the siding as if the deck were not even there.

How To Properly Flash A Deck Ledger Board - Helpful Advice for Home Construction Improvement