ᐅ Calculation of Buffer Volume for Rainwater Retention

Hello everyone,

I need to temporarily store rainwater on my property, and the required volume depends on the connected, impermeable surface area (3m³ (100 cubic feet) per 100m² (1,076 square feet) of connected, impermeable surface). Since the property is not very large, a retention basin is not an option, so I am considering a retention tank.

My question is: Does the connected, impermeable surface area only include the roof areas of the house and garage, or does it also include the terrace and driveway? Or does "connected" here specifically mean only surfaces that are actually connected to the tank via a gutter or similar system? I am not concerned about stormwater fees, just wanting to know how large my retention tank needs to be.

With a roof area of 180m² (1,938 square feet) for the garage and house, would I thus need a retention tank with a volume of 5.4m³ (191 cubic feet)?

Thank you very much

Paths, the terrace, and the driveway should also be included. Essentially, everything connected to a drainage channel.

The design plan states that the driveway to the garage will be drained through the green strip. Or what determines whether the terrace and courtyard driveway are drained through a channel drain or not? As far as I know, no channel drains are currently planned, and the slope runs from the garage toward the street.