Rainwater harvesting and the runoff calculation

Ask for a rainwater harvesting attenuation credit and the lead local flood authority will usually say no, and the reason is a single design assumption: the tank might be full when the storm arrives. Attenuation volume exists to hold water back during an extreme event, and a harvesting tank sized to hold water for reuse is, by design, trying to stay full. The two purposes are in direct conflict, and a calculation that counts the same cubic metres twice will be rejected.

There is a defensible partial credit, and it comes from designing two volumes in one structure rather than from arguing about one. That takes a slightly larger tank and a flow control, and it has to be shown on the drawings rather than asserted in the text.

What the attenuation volume has to achieve

The starting point is the discharge rate. On a greenfield site the requirement is normally to limit runoff to the greenfield rate for the relevant return periods, and on a brownfield site to achieve a betterment against the existing rate. The Defra non-statutory technical standards for sustainable drainage systems set out the peak flow and volume control expectations, and the lead local flood authority applies them alongside local policy.

The attenuation volume then follows from three inputs: the impermeable area draining to the system, the design event including the climate change allowance and an allowance for urban creep, and the permitted discharge rate. The calculation is run across a range of storm durations to find the critical duration for that combination, because the worst case is not always the longest storm.

Two things follow that matter for harvesting. The volume is sized for a rare event, which means the storage sits empty almost all the time. And the volume is only doing its job if it is available when the event happens.

That second point is the whole argument, and a Drainage Strategy which does not address it directly will come back with a question rather than an approval.

Why harvesting storage is not credited in full

The guidance approach, following the industry SuDS manual, is that storage provided for rainwater reuse should not be counted towards attenuation unless it can be shown to be available when needed.

The reasoning is straightforward. A harvesting tank fills from the first rainfall after a period of demand and stays substantially full until the demand draws it down. Design storms do not arrive at convenient moments, and the standard conservative assumption for attenuation design is that the reuse volume is unavailable.

There is a secondary reason that carries weight with reviewers. A harvesting system depends on pumps, filters and controls, and on somebody maintaining them. Attenuation storage designed as a void with a flow control has no moving parts. A regulator comparing the reliability of the two is not being unreasonable in treating them differently.

Our note on whether rainwater harvesting counts as attenuation deals with the policy side of the question. What follows here is the calculation.

Sizing for yield against sizing for storage

The two sizing exercises produce different numbers from different inputs, which is why a tank sized for one rarely suits the other.

Yield sizing starts from the roof area, the local annual rainfall, a yield coefficient for the roof surface and a filter efficiency, giving the annual volume that can be collected. That is compared with the annual non-potable demand, which for housing means WC flushing, external tap use and any communal irrigation or vehicle washing. Under the simplified approach in the harvesting code of practice, storage is then commonly set at around 5 per cent of the lesser of the annual yield or the annual demand.

Attenuation sizing, by contrast, starts from the impermeable area and the design storm, and takes no account of demand at all.

On a small housing scheme the yield volume is often a fraction of the attenuation volume, which is the practical reason the credit argument is usually not worth having. On a scheme with high non-potable demand, such as a building with extensive irrigation or process use, the numbers converge and a combined design starts to make sense.

Combined attenuation and harvesting tanks

The design that gets accepted puts both volumes in one structure with a clear separation between them.

1. A lower harvesting volume, drawn down by the pump to supply non-potable demand.
2. An upper attenuation volume above the harvesting overflow level, kept empty in normal operation.
3. A flow control on the outlet from the attenuation volume, sized to the permitted discharge rate.
4. An overflow route for exceedance, designed and shown on the drawings.
5. A high level alarm and a maintenance access arrangement.

Only the upper volume is claimed as attenuation, and only if the design keeps it empty. Where an active control system is proposed, releasing stored water in advance of forecast rainfall, the credit can in principle be larger, and the evidence burden rises accordingly: the control logic, the forecast source, the failure mode and the management arrangement all have to be described and secured.

Illustrative example: a scheme with 540 square metres of roof, a calculated attenuation requirement of around 28 cubic metres, and a non-potable demand supporting a harvesting volume of roughly 4 cubic metres. The accepted design was a single tank of about 32 cubic metres with the harvesting volume in the base and the attenuation volume above it, discharging through a flow control. No credit was claimed against the 28 cubic metres. The figures are illustrative and used to show the arrangement.

That is the honest position on most schemes. The harvesting system is a water efficiency measure that happens to share a hole in the ground with the attenuation, rather than a way of making the attenuation smaller.

Where water efficiency policy overlaps

Harvesting earns its place through water efficiency rather than through drainage, and the two policy strands are assessed separately.

Building regulations set a water consumption standard for new dwellings, with a tighter optional requirement that local plans can apply through a planning condition. Compliance is demonstrated either through a fittings approach or through the water efficiency calculator, and the calculator recognises a contribution from rainwater and greywater reuse.

So on a scheme in a water stressed area with the tighter standard applied, harvesting can be the measure that closes the gap when the fittings specification alone cannot. Say that in the sustainability case, and keep it separate from the drainage calculation so that neither argument contaminates the other.

Non-domestic schemes pick up credit through assessment methods and through local policy on water use, and the same separation applies.

Maintenance and the risk of abandonment

Regulators discount harvesting because they have seen it abandoned. Pumps fail, filters clog, UV lamps are not replaced, and in a block of flats the management company inherits a system nobody explained to them. The tank then sits full of water doing nothing for either purpose.

Three design responses help.

Keep the system simple, with as few components as the duty allows, and locate everything where it can be reached without scaffolding or a confined space entry.

Write the maintenance requirement into the management arrangements rather than into a handover folder, with the inspection interval and the replaceable components listed.

And design so that failure is safe. If the pump fails, the system should default to mains supply for the non-potable demand and the tank should still overflow through the flow control, so a maintenance failure does not become a flooding failure.

Claiming a partial credit defensibly

If you are going to claim anything, claim it like this.

State the attenuation volume required from the calculation, with the design event, the climate change allowance, the urban creep allowance, the permitted discharge rate and the critical duration. Then state the harvesting volume separately, with the yield and demand figures behind it. Then show the tank arrangement with levels, making clear which volume is claimed as attenuation and why it is available. Then describe the control, the exceedance route and the maintenance regime, and offer a condition securing all three.

Do not net the harvesting volume off the attenuation requirement without that structure. And where the site is in a flood risk area, the Flood Risk Assessment has to be consistent with the drainage design, because a reviewer reading both will notice a difference in the assumed storage. For a single dwelling the whole exercise is usually much simpler, as our note on drainage evidence for a single new dwelling sets out.

Note that the SuDS approval provisions in Schedule 3 to the Flood and Water Management Act 2010 remain uncommenced, so the approval route may change. Check the current position before relying on the existing arrangements.

This is general guidance, not legal advice, and drainage requirements vary between lead local flood authorities, so check the position with yours and take advice from a qualified drainage engineer before relying on it. PlanScope drafts a Drainage Strategy structured to UK planning policy, flagging every rate, area and volume it needs from your engineer. Enter your postcode to see which reports your site needs.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.