[freebeard]

I'd start by measuring your parking spot. I have a 14' car and it fits under the carport, but if I add a 3' boat-tail, somethings going to be sticking out in the rain.

Beyond your question: move the rear view mirrors to the tops of the front fenders, Moon disks in front and fender skirts in back, add double-walled side skirts [like basjoos] and clean up the underside with a rear partial pan that sweeps up into a
diffuser that ends at your new boat-tail.

I'd say about 18". Do you have a picture with the trunk or hatch half open?