In some countries and ...

In some countries and regions stormwater discharge into sewers is illegal, for the fact that the sewage handing system is usually compromised during storm events or massive infrastructure needs to be deployed to receive stormwater into sewers - the latter not always practical.

So stormwater from houses should be captured and used appropriately, there are several rainwater harvesting, collection and treatment options, systems and technologies available for this application, it should be encouraged wherever we are in the world. An above ground capture system located next to house (on top of septic tank) will most likely fit in your application

Regards