Sizing to policy, not to theory
Each council publishes its own permissible site discharge and site storage requirement. The theoretical optimum is irrelevant if it does not match the adopted policy — design to the policy and demonstrate compliance clearly.
Orifice plates and blockage
Small orifices block. Where the calculated orifice diameter falls below the council minimum, a different control arrangement is required, typically a weir and orifice combination with an accessible screen.
Access for maintenance must be shown. A tank that cannot be cleaned will not be maintained.
Buildability
Check the tank against the structural design early. Below-ground detention under a driveway carries vehicle loads, and a late change of tank location can force a footing redesign.