As rumours swirl around about the demise of the Code for Sustainable Homes, surely now is the time for the Government to take a step back from its ill-advised insistence on forcing rainwater harvesting systems into new dwellings and instead start to incentivise the decoupling of rainwater downpipes from existing buildings.

At its core rainwater harvesting is beautifully simple. You collect rainwater from a clean surface (i.e. a hard roof), filter, store and reuse. Practiced for centuries, rainwater is still used by millions of people around the world who don’t have access to a mains water supply. And in Germany, harvesting rainwater for non-potable and non-bathing purposes is seen as eminently sensible with approximately 60,000 systems installed a year1. And yet in the UK rainwater harvesting is becoming anything but simple. So what has gone wrong?

Code requirement for rainwater harvesting and SuDS

The original idea of the Code was that rainwater harvesting or greywater recycling would be required in homes built to Levels 5 and 6. In the absence of grants this would support the growth of a fledging industry and result in a gradual build-up of the required expertise. Another key requirement of the Code is that any surface water run-off from a new development that is greater than from the original site must be disposed of on site. It is a bold attempt to drive forward the implementation of SuDS solutions on new developments, protect receiving waters from pollution, minimise the risk of flooding and prevent other environmental damage in watercourses. If it is not possible to dispose of all the surface water run-off on site then attenuated flow into the sewers becomes allowable. However, full justification for attenuation as a first option must be provided and (crucially) rainwater harvesting for use back in buildings is classified as a SuDS solution.

Therefore, unless the new development is an urban infill site or on industrial land, post development run-off will always be greater. And, when ground conditions preclude the use of infiltration techniques, rainwater harvesting becomes the only possible SuDS solution and must be installed. But what we are doing here is mixing two different types of stormwater run-off. SUDS are designed to attenuate, treat and dispose of all surface water from a site. Rainwater harvesting is designed to collect rainwater falling on hard roof surfaces. The Code, as it stands, leads to rainwater being collected from all surfaces, including paved areas and green roofs, resulting in collected water of dubious quality.

As a consequence, highly engineered filters or complicated cleaning regimes are specified. The Hydro filter is a case in point. It is very efficient at filtering out nutrients within rainwater and was originally designed to be installed between road gullies and local receiving waters to prevent nutrient run-off into these waters. It is an excellent highways solution, but the filters are expensive, harder to maintain than standard filters and are over-specified when used to filter run-off from green roofs. I have heard of a site where ozonisation is being considered because the current stored rainwater is so contaminated and I have also heard Housing Association managers say that the rainwater harvesting systems that are installed to meet the Code are disabled virtually on handover due to the high cost of maintaining them.

Of course there are good examples of rainwater harvesting systems, that collect from hard roofs, are well installed, properly commissioned and with a suitable maintenance regime in place. But most householders will save more water by reducing their shower by five minutes a day than from a rainwater harvesting system installed in a house with a 50m2footprint (the size of most new build homes). 2

Simpler SuDS and rainwater harvesting where it makes sense

Of course disposing 100% of rainwater on site when ground conditions allow should be the ultimate aim. It can be done in a variety of ways; rain gardens, natural ponds, tree pits, rainwater for use on allotments are all low cost to implement and simple to maintain (though not maintenance free). Soakaways using crate systems under car parks and paved areas can take advantage of the natural permeability of the ground even on clay sites.

BREEAM can continue to incentivise rainwater harvesting in non domestic buildings. Gardens with space for vegetable plots (surprisingly rare in most new developments) should require “super butts” of 500 litres storage. And lawns over a certain size (I’m thinking large lawns here) should require sub-surface irrigation systems fed from rainwater. Self builders could be incentivised to install well designed rainwater harvesting systems with high quality components, providing the need for maintenance is clearly spelt out and the water savings not overstated.

Dealing with stormwater run-off is vitally important and from existing buildings not just new build. Let’s decouple all our buildings from the stormwater stream to provide extra capacity in sewers, to reduce the incidents of surcharging of drains and localised flooding incidents and to limit sewage spills through CSOs. Let’s engender smarter thinking and smarter design that understands that small (and simple) is often best. Please let’s stop installing over-engineered rainwater harvesting systems into homes that within a few years of being installed are no longer in use; instead let’s harness the rain to water flowers, not to flush WCs.


  1. “60,000–80,000 compared to 4,000 in the UK”. Lutz Johnen. Chair of UKRHA. Email correspondence Feb 10th 2013
  2. 50m plan roof area in most new social housing, (less if flats). 750 x 0.9 x 0.9 x 50 = 30,000 litres/year = 83 litres/day. = 21 litres per person (4 person occupancy). Reduce shower time by 3 minutes in a shower with a flow rate of 7 litres/minute = 21 litres.

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