A rain barrel or small tank set up under a downpipe is one of the simpler DIY projects in a garden, and it does real, measurable work: every litre collected off the roof during rain is a litre that doesn't come out of mains water, or a bucket, during the next dry stretch. It pairs naturally with water-wise plant selection, covered separately on this site, as the supply side of the same approach: use less, and catch more of what falls for free in the first place.
Sizing depends mostly on your local rainfall pattern rather than the size of your garden. In a region with reasonably consistent year-round rain, a standard 240-litre rain barrel is often enough to keep topping up through a dry week. In a region with genuinely dry summers, or a garden with high water demand, a larger tank, 500 litres or more, holds a meaningful reserve through a longer dry stretch instead of running dry after the first sunny weekend.
The setup itself is a straightforward downpipe job. A rainwater diverter fits into a cut gap in an existing downpipe and redirects water into the tank instead of straight down the drain, and simply twists off once the tank is full, sending the rest back down the pipe as normal. Stand the tank on a firm, level base near the downpipe, and raise it on a stand if you can: a stand between roughly 300mm and one metre gives enough gravity-fed pressure for a hose, and enough clearance to fit a watering can or bucket under the tap. Going higher than a metre starts to risk needing building consent, so there's a real ceiling on how tall to go without extra paperwork.
A rainwater diverter fits into a cut gap in an existing downpipe and redirects water into the tank instead of straight down the drain, and simply twists off once the tank is full, sending the rest back down the pipe as normal.
Beyond the tank, diverter and stand, the only other parts worth having on hand are a garden hose, thread sealing tape for a leak-free tap fitting, and a debris filter at the inlet, which is worth checking and clearing periodically so leaves and grit don't block the flow into the tank.
A couple of safety and maintenance habits matter more than they might seem. Keep the tank covered, both to stop debris and mosquitoes getting in and, more importantly, for safety if young children are ever around the garden. And check that debris filter regularly rather than only when water flow noticeably slows, since a partly blocked filter can quietly cut collection efficiency well before it stops water flowing altogether.
A basic rain-barrel setup like this doesn't need a permit, and most home gardeners can put one together themselves in an afternoon. The one exception is a system plumbed into the house's mains water supply rather than kept as a standalone garden tap, which does need a registered plumber. For a simple downpipe-to-garden-tap setup, that's not a concern, and it's realistically one of the highest-value weekend projects available for a garden heading into a drier summer.