Rainwater harvesting and the drainage credit

A rainwater harvesting drainage credit is the claim that a storage tank installed to supply non-potable water also provides attenuation, so the scheme needs less dedicated storage to meet its discharge rate. Most lead local flood authorities will not accept it, and the reason is simple arithmetic. Attenuation only works if the volume is empty when the storm arrives, and a tank sized to supply a household is designed to be full.

That does not make harvesting pointless. It makes it a water efficiency measure that happens to sit in a tank, and the credit has to be earned through a specific design rather than assumed.

What harvesting does to run-off

A harvesting system intercepts roof water, holds it, and releases it through a tap or a WC cistern rather than to the drainage network. Over a year that reduces the total volume leaving the site, sometimes substantially.

Over a single design event it may do nothing at all. The calculation that matters for a drainage strategy is the peak discharge rate in the 1 in 30 and 1 in 100 year events with a climate change allowance, and the storage available to throttle it down to the agreed greenfield rate. A full tank contributes no storage to that calculation, and an overflowing tank passes its inflow straight through.

So there are two different performance questions with two different answers, and conflating them is what gets a strategy rejected.

Why authorities discount it

Four objections come up consistently, and they are all reasonable.

The tank state at the start of the event is unknown. Without a control that guarantees headroom, the design has to assume the worst case, which is full.

Demand is seasonal and inverse to need. Garden irrigation, which is the largest domestic demand, falls to nothing in the winter months when prolonged rainfall events are most likely. A system sized on annual average demand overstates winter drawdown.

Control is lost at occupation. A private system in a single dwelling can be switched off, bypassed or left with a failed pump, and no condition realistically prevents it. For a 40-dwelling scheme that means 40 independent points of failure.

And there is no mechanism to verify it. Attenuation storage with a flow control can be inspected. Whether a tank was half empty on the night of a storm cannot.

Sizing for supply against sizing for attenuation

The two calculations pull in opposite directions, which is the whole difficulty.

Supply sizing follows BS 8515, the code of practice for rainwater harvesting, which offers a simplified approach based on a proportion of the lower of annual yield and annual demand, and a more detailed approach using daily or monthly time series. Yield depends on roof area, surface coefficient, filter efficiency and local rainfall. Demand depends on occupancy and which fittings are connected.

Attenuation sizing follows the drainage calculation: greenfield run-off rate, the critical duration storm, and the volume needed to hold the difference. Which rainfall dataset you use changes the answer, and our note on which rainfall dataset to cite in a drainage calculation covers the choice.

Run both. Then size the tank for supply and the attenuation separately unless the design below applies.

Combined tanks and how a credit can be earned

Where a credit is accepted, it is almost always for a tank with two distinct zones: a retention volume at the base serving the supply, and an attenuation volume above it with a throttled outlet to the drainage network.

For that to count, the design has to show four things.

1. The attenuation zone is above the highest level the supply system can fill to, with the supply draw-off set below it.
2. The attenuation zone has a controlled outlet sized for the permitted discharge rate, and the zone drains down within the period the authority's standards require.
3. The retention volume is excluded from the attenuation calculation entirely, so no part of the credit depends on demand.
4. Overflow and exceedance routes are shown for the event that exceeds the design, with the flood route away from buildings.

Done that way, there is no credit in the sense of borrowing supply volume. There is a single structure doing two jobs with a physical separation between them, which is a different and far more defensible proposition.

Active control systems, which release stored water in advance of forecast rainfall, are used on larger schemes. They introduce a dependency on power, telemetry and a forecast feed, and an authority will want to know what the system does when any of those fail, in the same way it tests a partially blocked outlet, as discussed in modelling a partially blocked flow control.

Water efficiency policy and the real benefit

The stronger case for harvesting is water consumption rather than drainage.

Part G of the Building Regulations sets a water efficiency requirement for new dwellings, with a tighter optional standard that applies where a planning condition imposes it. A number of authorities in water stressed areas impose that condition, and some regional policy goes further still. Harvesting contributes to meeting the tighter figure alongside low-flow fittings and efficient appliances.

In water neutrality and water stressed catchments the argument is stronger again, because the question is total abstraction rather than peak flow. Where supply capacity itself is the constraint, a Water Supply Assessment is the document that carries the evidence.

Present harvesting there, in the water efficiency section, where it earns credit that is actually available.

Maintenance and failure modes

Harvesting systems fail quietly, and the failure modes are worth stating in the submission because they shape the conditions.

Leaf litter and silt blind the filter, which reduces yield and eventually diverts everything to overflow. Pumps fail, and in a system without mains backup the occupier simply loses the supply. In a system with mains backup, the backfeed arrangement has to maintain the required air gap under the water fittings regulations, because a cross-connection between harvested water and the mains is a public health matter rather than a plumbing preference.

Stagnation in an oversized tank with low demand produces odour and biofilm. Above-ground tanks and exposed pipework freeze. And the first flush off a roof carries the highest concentration of debris, so a diverter or a self-cleaning filter is part of the specification rather than an upgrade.

Set out who maintains what, at what frequency, and what the householder is handed at completion.

What to claim in the strategy

Be specific and claim less than you could.

State the harvesting system, its tank volume, the roof area connected, the fittings served and the sizing method used. Then state plainly whether any attenuation credit is claimed. If none is claimed, say so, and show the attenuation storage as a separate calculated volume. If a credit is claimed, show the two zones on a section with levels, the outlet control, the drain-down time, and a sentence confirming the retention volume is excluded.

Check the authority's published guidance first. Several lead local flood authorities state their position on harvesting explicitly, and a strategy that contradicts a published local standard will come back whatever its merits. Put the whole picture in the Drainage Strategy rather than splitting it between documents.

Enter your postcode on PlanScope to see which drainage and water documents your site is likely to need.

This is general guidance, not legal advice: check the position with your local planning 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.