Our church just barely made it for the new rules for building a new building. We needed a larger retention pond to account for the new building (and therefore less wooded area for rain to naturally seep into).
What we ended up doing was putting in an underground vault in addition to the existing pit. The vault is under the parking lot, and something like 30’x100’x20’.
The new rules would have prevented us from building the new building as they called for even larger storage capacity - larger than what the property had space for! The site still has more wooded/grassy area than paved or building area. Goofy.
It is expensive, but would your city have accepted the new porous asphalt? It allows the water to seep through, and therefore acts more like natural turf.