"I'm guessing you're not a structural engineer."
Share your design experience with us. Add the weight of 2' feet of snow (a typical requirement), along with 20% of that roof having the increased weight of planters and 12" depth of wet soil, plus the weight of vegetation itself, and you have a much heavier roof, which in turn requires stronger walls, and a larger foundation, and so on.
Now pass those costs along to the owner and tenants, and explain the inflated costs are due to decisions made by people who don't consider all of the ramifications.