Rainwater harvesting in the UK: a practical home guide

We get a fair bit of rain in this country. It's practically the national personality. So it always seems a small tragedy to watch it thunder off the roof, down the drain and away, while you're stood there in August paying your water company for something the sky was handing out for free in April. Rainwater harvesting is just the fix for that: catch it, store it, use it.

Is rainwater harvesting actually worth it in the UK?

Short answer: yes, especially for the garden, and especially if you're on a water meter.

Rainwater is soft, free of chlorine, and slightly acidic, which most plants prefer to hard tap water. Ericaceous things like blueberries and rhododendrons genuinely do better on it. It's kinder to a water butt full of tomatoes than the stuff coming out of your tap in a limescale-heavy area.

The money side is more modest, so let's be honest about it. A single butt won't slash your bill. But if you're metered and you water a decent veg plot, greenhouse or a thirsty lawn through a dry spell, the savings add up over a few summers, and a butt pays for itself. During a hosepipe ban, a full butt is worth its weight in gold. The real win is resilience: you're not left staring at a brown garden because the tap's been turned off.

There's a quieter benefit too. Every litre you catch is a litre that isn't hammering into the drains during a downpour, which eases the load on stretched Victorian sewers. Small thing, but it counts.

How much rainwater can you actually collect?

Here's the one bit of maths worth knowing, and it's genuinely simple. For every millimetre of rain that falls on every square metre of roof, you collect roughly one litre.

That's it. So work out your roof's footprint (the area it covers on the ground, not the sloped surface), multiply by the rainfall, and knock off about 20% for splash, evaporation and the first flush you'll want to divert away.

Across the UK the average is about 1,163mm of rain a year, though the range is wild: parts of the south-east and East Anglia scrape under 700mm, while the Lake District and Snowdonia get metres of the stuff. Even at the dry end, a modest shed roof fills a 100 litre butt in one wet week. You are not short of raw material.

Water butt or a proper tank: which do you need?

This comes down to how much you'll actually use, and how much space you've got.

A water butt is where nearly everyone starts, and rightly so. A basic 100 to 210 litre butt costs around £30 to £40, clips onto a downpipe with a diverter kit in an afternoon, and covers most garden watering. If one runs dry too often, link two or three together with a connector kit so they fill in sequence. It's cheap, it's reversible, and it teaches you what you actually get through.

Step up and you're into larger tanks. A 1,000 litre IBC (those caged plastic cubes) can often be had for £80 to £150 and is a favourite on allotments and smallholdings. They're not the prettiest thing in the garden, but they hold serious volume for the money. Tuck one behind a shed and it'll see a greenhouse through a dry fortnight.

At the top end sit whole-house systems: a large tank, often buried, feeding rainwater indoors through its own pump and pipework. These are a proper project, frequently £2,000 to £5,000 or more installed, and they make most sense in a new build or a big renovation where you can lay the pipes before the walls go up. Retrofitting into an existing house is doable but fiddlier and dearer.

Start with a butt. Live with it for a season. If it's bone dry every August and you're still lugging the watering can to the tap, that's your cue to go bigger.

How do you use the water once you've caught it?

The garden is the obvious home for it, and honestly, for most people that's the whole story: fill the can, feed the beds, done. No treatment, no worries. A watering can or a simple gravity-fed drip line off the butt does the job.

Push further indoors and rainwater is perfect for the jobs that don't need drinking-quality water. Flushing the loo and running the washing machine between them account for a big slice of household use, and neither cares whether the water came from a reservoir or your roof.

1. 1

   Start at the downpipe

   Fit a rain diverter into an existing downpipe. It skims water into your butt or tank and lets the overflow carry on down the drain once full.

2. 2

   Add a first-flush diverter

   For anything beyond a basic garden butt, send the first few litres of each shower (the dirtiest, carrying roof grit and droppings) to waste before the clean water starts filling your store.

3. 3

   Filter before storage

   A leaf filter or fine mesh at the tank inlet keeps out debris and stops the store turning into a compost bin.

4. 4

   Pump and plumb for indoor use

   Feeding the loo or washing machine needs a pump and dedicated non-potable pipework, kept entirely separate from your mains. This is the notifiable, get-a-pro stage.

5. 5

   Keep a mains backup

   Any indoor system needs an automatic mains top-up for dry spells, and it must connect through an air gap so rainwater can never flow back into the drinking supply.

For a deeper look at running a home mostly or wholly off collected and stored water, see our sister guide to off-grid water systems in the UK.

How do you keep stored rainwater clean, and cope with winter?

Stored rainwater goes stagnant if you neglect it, but keeping it sweet is mostly common sense. Keep light out, since a dark, covered butt won't grow algae the way a clear one will. Fit a lid to keep leaves, midges and curious wildlife out. Once a year, empty it, give it a scrub, and clear the gutters and any filters that feed it. Use the water regularly so it turns over rather than sitting for months.

Winter's the other thing people forget. A butt can freeze in a hard UK frost, and a solid block of ice will happily crack a cheap plastic one. You rarely lose the whole lot, but the tap and the top few centimetres are the weak spots. Leave a little headroom at the top so expanding ice has somewhere to go, lag the tap, and on a genuinely icy morning just draw from the mains rather than wrestling a frozen butt. Larger tanks and buried systems hold their temperature far better and mostly ignore the cold.

Is it legal, and can you drink it?

The legal picture is friendlier than people fear, so let's clear it up.

Catching rainwater off your own roof needs no abstraction licence. The Environment Agency says so plainly in its regulatory position statement: harvested rainwater isn't regulated as long as it's only rainwater and you're not combining it with water drawn from a stream, ditch or the ground. Fill as many butts and tanks as you like. That's your rain, and it's yours to keep.

The rules only tighten when rainwater comes indoors. Under the Water Supply (Water Fittings) Regulations 1999, any non-potable supply plumbed into the house must be kept completely separate from your mains drinking water. That means a physical air gap on any mains top-up (not just a valve, since valves can fail), clear labelling on the pipework so no future plumber ever cross-connects the two, and notifying your water company before the work starts. Look for WRAS-approved fittings and use a competent installer. This is the one part of rainwater harvesting worth doing properly rather than winging it.

So there it is. Start with one butt on a downpipe this weekend, watch how fast it fills, and let the garden tell you whether it's worth going bigger. The rain's coming either way. You may as well catch some of it.