If you're over 25,000 square feet and you need to replace your roof, on an existing building, the ordinance will apply!  The problem is that most existing roofs are designed to support 30-40 pounds per square foot, not the 80-120 psf that Green Roofs weigh, so, in addition to paying for a new roof and a bunch of new plants, you get tho reinforce the existing roof structure to support the higher loads!