Consider putting in what I did ....I have a 6500L underground storage tank.
All of my downpipes go to underground surface water drain as normal ... what I have is after final gulley connection, I have the pipe diverted to tank ... so it collects all roof water.
I added a large outbuilding last year and have also added that to take feed.
System is automatic .. as long as water is >10% in tank the system provide mains pressure recycled water, if tank drops below 10%, it auto tops up with mains water ... saving messing about with manual divert valves etc. I plumbed in a 3rd pipe run in house .. So I have Hot, Cold & recycled ... the recycled goes to the WC's washing m/c and outside taps.
At least 30% of your water is used in flushing your loo .... so I save well over that.
You can buy the bits individually, I bought it as a System from ...