Rainwater harvesting in a drainage and water strategy

Rainwater harvesting earns its place in a drainage and water strategy as a demand reduction measure, not as attenuation storage. That distinction decides how you present it, and getting it wrong is the fastest way to have a strategy returned by a lead local flood authority.

A harvesting tank reduces the volume of mains water a building uses. It does not reliably reduce peak run-off, because on the day of the design storm the tank may already be full.

What harvesting contributes to water efficiency

Part G of the Building Regulations sets a water efficiency requirement for new dwellings, expressed as litres per person per day, with a tighter optional requirement that a local planning authority can impose by condition where the adopted plan justifies it. Compliance is demonstrated using the published water efficiency calculator.

The calculator works mainly on fittings: the flush volume of the WC, the flow rate of the taps and shower, the capacity of the bath, the consumption of the dishwasher and washing machine. A rainwater or greywater system attracts a credit within the methodology, calculated on the basis the methodology sets out rather than on your own estimate of yield.

That credit is real and sometimes decisive on a scheme that is a litre or two over the target. It is rarely the least costly way to get there. Specifying a 4/2.6 litre dual flush WC and a lower flow shower usually costs less than a tank, a pump, a filter and a second pipework circuit.

So the honest framing for a design and access or sustainability narrative is that harvesting supports the water strategy alongside fittings and, in water stressed areas, alongside the wider demand case set out in a Water Supply Assessment.

Whether harvested storage counts as attenuation

Almost never, and the reason is simple. Attenuation storage has to be available when the storm arrives. A harvesting tank is designed to hold water, so its available volume at any moment depends on recent rainfall and recent demand, neither of which you can guarantee.

Drainage guidance, including the widely used CIRIA SuDS manual, treats retained volume in a harvesting tank as unavailable for attenuation unless the design ensures it is empty before the event. That is why most lead local flood authorities allow no credit at all for a standard system.

There is a design that does earn credit. A dual-purpose tank splits the volume: a lower retention volume that stores water for use, and an upper attenuation volume above it that is emptied by a controlled outlet after each event. The attenuation volume is always available because it drains down, and the calculations can show that.

If you propose one, show the split explicitly on the section, state the control device and its discharge rate, and run the storm calculations on the attenuation volume alone. Present the retention volume as having zero attenuation benefit. That approach survives scrutiny, and it is the approach that sits well with what the lead local flood authority checks in a drainage strategy.

Sizing against demand rather than roof area

The common mistake is to size the tank from the roof. A large roof produces a large yield figure, the yield figure produces a large tank, and the tank never empties.

Size it from the demand it will serve. For a house that means the WC flushes, the washing machine if it is connected, and outside taps. For an apartment block it means the same list multiplied by occupancy, plus any landscape irrigation and communal cleaning.

The standards give two workable approaches. BS 8515 remains the familiar UK code of practice for rainwater harvesting systems, and BS EN 16941-1 covers on-site non-potable water systems using rainwater. Both point in the same direction: calculate the yield from the collection area and the local rainfall, calculate the demand from the intended uses, and size the storage as a proportion of the smaller of the two over a defined period.

Two sanity checks are worth running.

• If the tank would take more than a few weeks of typical demand to empty, it is too big
• If the connected demand is only WC flushing in a two-person flat, the payback on a full system rarely justifies it

State the occupancy assumption in the strategy. A reviewer who cannot see the assumed occupancy cannot check the sizing, and will ask.

Maintenance, water quality and backflow

Harvested rainwater is non-potable and must be kept separate from the wholesome supply. The Water Supply (Water Fittings) Regulations 1999 govern that separation, and a rainwater system is treated as a high risk fluid category, which means a physical air gap at the point where any mains top-up enters the system rather than a valve.

The other water quality elements are a leaf and debris filter before the tank, a calmed inlet to avoid disturbing settled sediment, an overflow with a trap and a rodent guard, and clearly marked non-potable pipework and outlets.

Maintenance is the part that fails in occupation. Filters need cleaning, tanks need periodic inspection and desludging, pumps need servicing, and mains top-up controls need checking. On a block of flats that duty falls to the management company, and it belongs in the same conversation as SuDS adoption and maintenance, because the same party usually ends up holding both.

Where a council conditions the installation, expect the condition to require retention and maintenance in perpetuity, so the maintenance schedule needs to be written before the condition is discharged.

Where harvesting helps a water neutrality case

In catchments where water neutrality applies, and in areas of serious water stress, demand reduction carries weight it does not carry elsewhere. Harvesting becomes one of the measures an applicant can point to, alongside high efficiency fittings, leak detection, metering and, where the policy requires it, offsetting.

Be careful about how much you claim. A neutrality case is arithmetic, and the arithmetic has to hold. Credit only the demand the system will actually serve, using the same methodology the authority accepts, and do not assume the system yields through a dry summer without storage to match.

The wider position on what applicants have to demonstrate is set out in the guidance on water neutrality, and the harvesting element should be consistent with it rather than making a separate claim.

Presenting it honestly in the strategy

Put the harvesting section in one place in the Drainage Strategy, and give the reviewer everything needed to check it.

1. The collection area, with the roof types and any excluded surfaces.
2. The demand calculation, with the occupancy assumption and the connected uses.
3. The tank volume and the basis on which it was sized, naming the standard followed.
4. A section showing retention and attenuation volumes separately if the tank serves both.
5. The attenuation credit claimed, which for a standard system is zero.
6. The maintenance regime and the party responsible for it.

And say plainly what the system does not do. A strategy that claims a harvesting tank solves both water efficiency and surface water attenuation reads as overselling, and the reviewer then examines every other number in the document. One that states the attenuation credit is nil, and then shows the separate SuDS train that provides the attenuation, gets approved.

Browse the drainage and sustainability report types on PlanScope to see what a scheme in a water stressed area typically needs.

This is general guidance rather than legal advice. Check the position with your local planning authority, the lead local flood authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.