In many cities where more that 40% of the Erf is covered with building or hard surface the cities require some form of rainwater/storm water attenuation system. Traditionally it was require to install a soak pit with a volumetric size of 1 cubic meter for every 40 sqm of hard surface area.
An alternative has now been established where homeowners can install rainwater tanks. They can claim 60% of the volume. An example A property 1000sqm is covered by 500sqm of hard surface. It will require at least (500/40) = 12.5 cubic meters for a soak pit. By installing a (12.5/60%) = 20000l rainwater tank they would not have to install a soak pit and can install a rainwater harvesting tank and use the water for general purposes.
It is not only cheaper to install rainwater tanks but the owner will save significantly on his water bill
If you need information on the above standard see this link