The rain from my back roof used to drain directly into the drain and from there into the septic tank in next doors garden, we are at the end of a down hill cul-de-sac and just too low for the main sewer. There was a diverter that fed some of it into two 200 liter butts, but they wouldn't always fill, and too much water going through a septic tank is not a good thing.
I ran the pipe from the gutter direct into one barrel I left on the patio and moved the other butt half way down the garden, that instantly made the patio look better. I connected these with a copper pipe across the patio at the foot of the house wall and then a MDPE pipe buried under the lawn to the lower one. When the lower one fills there is still about a third of the top one empty so it can absorb a sudden influx of heavy rain. I then brought in a third barrel at the very bottom of the garden. I am still experimenting with that to get it almost to the level of the one above, I have it on a solid base of breeze blocks with paving stones on top. Ultimately any excess I can't contain will overflow at the bottom of the garden where there is a large oak tree, it is very dry under the tree's canopy.
The front roof runs into two connected 100 liter buts and overflows into a soakaway in the front garden, eventually I hope to get it connected to the system, really torrential rain can overflow the top butt before it can get down the pipe and it would be good to connect it to the soakaway, as well as being good to have that extra water. There is also a 100 liter butt collecting from the large greenhouse, that pretty much all goes into the greenhouse and rarely overflows, if it does a hose takes it down to the dry area.
The people before us didn't bother with the garden much, but the original owners had a veg patch down the end, unfortunately the oak has grown in the last forty or so years, so the best bit of earth that was cultivated is now very dry, I have started new beds further up, but it is solid clay and two years hard work piling in sand and compost and burning the clay are only just now beginning to have an effect, so I do need to water that dry bit. I have put in a second, smaller greenhouse over part of it, if I am going to have to water anyway ... but I still have kale, chard, lettuce, tomatoes, and mangetout that all need constant watering, as well as the toms, cues, and chili in the greenhouse.