The project I’m referring to is a LEED 2009 New Construction project, and has already been sent for the design review and received back with review comments.

Under the Storm Water design-Quantity Control credit, we have received a comment mentioning our rainwater harvesting tank is not enough to manage the rainwater.  However, we realized that we had assumed/used an incorrect impermeability value for the original site (which is 0.2).

The site used to be an unpaved parking lot with gravel on top of the hardened soil. Therefore the impermeability value must be much higher. Since we are required to manage the storm water runoff difference between the original site and the site after construction (we have chosen Option 2, Case 1), this low impermeability value has been a huge disadvantage for us.

I was wondering whether it’s possible to change the impermeability value at this stage. If so, do I need to provide any kind of a proof? Since the site has been built already, it’s impossible to do any kind of a soil test to verify the conditions of the original site. However, we have photos of the original site and have included in the initial site assessment report, which we have sent with the original design submission.

Could someone please let me know?

Thanks in advance,