The concept couldn’t be simpler. Instead of letting rainwater from the roof and paved areas around the building run to waste, it is collected, filtered and reused as a substitute for treated mains water in non-critical applications. Clearly, water captured in this way cannot be used for drinking, bathing, showering or where there is any possibility of accidental ingestion However, captured rainwater is perfectly suited for general cleaning, toilets, laundry and watering plants. In fact, the absence of chemical additives and retention of natural properties is a positive benefit to plants.

Actual savings can be difficult to quantify as they depend on the level of rainfall locally and the charges levied by individual water companies. But the UK Rainwater Harvesting Association (www.ukrha.org) estimates that commercial projects are likely to see a payback within two to five years. After that, it’s all money in the bank. The level of rainfall in the UK varies greatly – according to the Met Office, East Anglia has as little as 500mm of rain each year, but this figure is ten times higher in the Western Highlands of Scotland — so savings will obviously vary across the country too. Building size is another factor in determining savings. The roof will be the main gathering area, possibly augmented by terraces and other paved areas nearby. Ignore car parks as these may carry pollutants. In some cases, particularly with multi-storey buildings, the water yield may seem too small in relation to demand to justify full scale harvesting. But even in these cases, installing butts on rainwater down pipes can provide water for plants as well as reduce demand for mains water and the effects of storm surges.