Well, we have done quite a bit of research and our solution for stormwater neutrality problems don't seem to be too onerous. At this stage we understand that we need to retain approximately 3000 litres of water on site in tanks, with a slow realease to the mains system. Basically standard stormwater pipe diameters into the tanks 80mm then out through a 30mm pipe. The intenstion is just to slow the slow into the stormwater to decrease load on the system when it rains. Simple. We are also going to look into the options of retaining more water. At the very least to use as grey water in the garden, but also possibly using as grey water into the toilets.