[osolemio]

With the rain water collection tank in the ground, including a top-up system to fill city water in it when it gets toward empty, is the best way to keep "watertight firewalls" between the two systems.

There is no need for a tank in the attic. There are already pumps made just for the purpose - keeping pressure in the line on demand. They are quite simple, actually. When the pressure gets too low, the pump starts, and then it stops when it gets too high again.A small buffer tank prevents the water pressure from cycling although some pressure and flow variation can be expected if the pump is too powerful. This is mainly a concern if you try making a system from scratch, and not buying a unit made for the purpose.

As city water is still quite cheap many places, ready made units can have a hard time to complete, when you calculate how many years to pay back the system. I am going to install it in any case (eventually) at least for the reason that I think it is utterly foolish to flush toilets with drinking grade quality city water ...