Stormwater for residential lot

Need a storm water plan for residential building permits by a licensed engineer in North Carolina. Plan must be approved by the town and account for and storm water that is over the maximum. 1. The collection method to catch and retain the water that is over the 25% maximum impervious area of land usage. The example the town gave was the collection of the downspouts to type of pipe and aggregate and the location of the containment site. I am going to get with my surveyor as he did not include the driveway (20x35) and the back porch is not to be covered. I came up with a total impervious area of roughly 3362 for the house drive and porch, Total lot area of 9600 and 25% of that is 2400 sq. So from what I understand we need to account for the retention of roughly 1000 sq feet of the impervious area storm water. I would think that this could be accomplished by collecting the back downspouts of the house. Please review the site plan and submit timeline, cost, and proof of credentials.