how is this credit calculated? can in include existing floor slab and deck? we are leaving a portion of the exiting walls that are about 20ft tall and our ceilings will be at 9'. can i calculate the full height of these walls and both sides?  Is this 50% of all existing or is it compared to new construction. Our project is on the second floor and we will be removing the majority of the existing walls but most of the new walls will be demountables.