Of course it is generally legal to collect rain from your roof, assuming local HOA rules allow it. But you won’t likely get much quality water from that. Better would be greywater collection and recycling.
in some states rain water belongs to someone else -- the most surprising thing I've learned this whole month...
I was reluctant to state it completely because California's constitution itself limits water rights. It would be unconstitutional to cover your entire property with a giant roof and capture all the water, thereby eliminating the creek downstream of your property and denying the downstream users of their riparian rights. However this is not the use I believe the OP was looking at.
Yes. They are not as useful as you might hope since California's dry season lasts six months, so they will go empty when you need them the most.
I guess it depends on how much water you collect....
I believe it's illegal in the Bay Area, I seem to recall someone getting in trouble for doing so.