I want to reduce our metered mains water usage. So I'm
considering using rain water collected from the roof via a
huge water butt outside for flushing the downstairs toilet.
I've heard of others that have done this. I just wondered
what issues I need to think about?

Has anyone else here done this or know of any associated
plumbing diagrams?

Ideally if the butt runs dry then the supply needs to
switch back to mains water (either manually or
automatically).

Presumably the rain water would need to be filtered to
remove any small debris that could block the ballcock
valve?

The rain water must be plumbed in so there is no
possibility of it feeding back into the mains supply.

I need to check for any associated building laws (here in
France).

Advice or suggestions anyone on this project?
--
David in Normandy