Interpretation for Underground Stormwater Retaining System

Good morning - We have a project that is greenfield. We are seeking some clarification on whether our underground stormwater retaining system falls under the allowance of 1) 15' beyond...main utility branch trenches OR 2) 25' beyond constructed areas with permeable surfaces. Our stormwater retaining system is overlain with permeable landscaping. It is on the utility side of the water quality unit, but is holding water on behalf of the site to restrict the flow of water to the utility. The system is made up of 36" pipe, but is connected to the local municipal system with 12" pipe. We just want to make sure that we are applying the correct site disturbance limits as we enter the final stages of design. Would you be willing to help clarify that we are correct in applying the 25' condition? Many thanks for your expertise - Candice Rogers