The drainage hierarchy and justifying a sewer connection

Surface water has to be dealt with in order of preference, and a connection to a sewer is the last option rather than the default. The surface water drainage hierarchy puts discharge to the ground first, then to a surface water body, then to a surface water sewer, and only then to a combined sewer. A drainage strategy that starts at the sewer and works backwards gets rejected, because the authority and the lead local flood authority both want to see each higher option ruled out on evidence.

Where the order of preference comes from

The hierarchy appears in national planning guidance on flood risk and in the non-statutory technical standards for sustainable drainage, and the same order sits in the Building Regulations drainage requirements. Different documents, consistent sequence.

The logic is straightforward. Water returned to the ground near where it fell mimics the natural situation, keeps flows out of the piped network and supports baseflow. Water sent to a combined sewer travels to a treatment works, consumes capacity that was built for foul flows, and contributes to spills in heavy rain.

So the hierarchy is an environmental sequence rather than an administrative one, and the evidence to depart from a step has to be about the site rather than about convenience or cost.

Ruling out infiltration

Infiltration is the first option and the one most often dismissed without testing. A statement that the ground is clay will not do.

The evidence an authority expects starts with infiltration testing by the recognised soakaway method, carried out at the depth and location of the proposed feature, with the results reported as a measured rate rather than as a conclusion. Add groundwater monitoring where the water table may be high, because a soakaway sitting in groundwater does not work and the standards require a vertical separation.

Four reasons for discounting infiltration are accepted where they are evidenced. A measured rate too low to drain the required volume. Groundwater too close to the base of the proposed feature. Contamination, where infiltrating water would mobilise a pollutant, which engages the source protection and groundwater position. And ground stability, including made ground, soluble strata and sloping sites where infiltration would create a slip risk, which is the problem explored in managing run-off on a steeply sloping site.

Proximity to foundations and to neighbouring structures is a design constraint rather than a reason to abandon infiltration, since it usually moves the feature rather than ruling it out.

Discharging to a watercourse

Where infiltration is properly ruled out, a surface water body is next. This is where the consents multiply, and the planning permission is only one of them.

You need the right to discharge, which means either riparian ownership of the bank or the consent of the owner, recorded in writing. Works in or over an ordinary watercourse need land drainage consent from the lead local flood authority, and works affecting a main river need a permit from the Environment Agency. None of those is granted by the planning permission, and all of them can be refused.

Then there is the outfall itself, which has to be designed rather than drawn: headwall, invert level set above normal water level, scour and bank protection, and a check that the outfall does not surcharge in a flood event. That detail is set out in outfall design, bank stability and scour protection.

Receiving capacity matters too. A small ditch that already floods will not absorb an extra flow, and the lead local flood authority may restrict the rate well below what the hydraulics would allow.

Connecting to a surface water sewer

A connection to a public surface water sewer is the third option. The right to connect for surface water drainage comes from the Water Industry Act 1991, and the sewerage undertaker can require the connection to be made in a particular manner and can refuse where the sewer would be overloaded or the mode of connection is unsatisfactory.

So the sequence is to make a pre-development enquiry to the undertaker, giving the proposed discharge rate and volume, and to get a written response on capacity and the point of connection. That response is the evidence the authority wants, and a drainage strategy asserting that the sewer has capacity without it is incomplete.

Expect a restricted rate. Undertakers commonly require attenuation so that the discharge matches a greenfield equivalent or a stated maximum, which pushes the design back into storage sizing and critical duration testing.

Why a combined sewer is the last resort

Putting clean rainwater into a pipe carrying foul flow is the least desirable outcome for everyone. It uses treatment capacity, increases the risk of spills during storms and raises operating costs.

Undertakers and lead local flood authorities both resist it, and where it is unavoidable the rate is usually restricted hard, with on-site storage and sometimes a requirement to remove an equivalent existing connected area elsewhere. Where the development is a conversion with an existing combined connection, the position is more arguable, though authorities will still look for a reduction rather than continuation.

Foul and surface water should be separated at source in any new scheme, whatever the eventual destination, and the foul side has its own capacity question addressed through a Foul Sewage Assessment.

Documenting each step

The deliverable is a short, auditable sequence rather than a narrative. A Drainage Strategy that gets through first time usually sets it out in this order.

1. Contributing areas and the proposed discharge destination.
2. Infiltration testing results, with the method, date, locations and measured rates.
3. Groundwater, contamination and stability evidence where infiltration is discounted.
4. Watercourse investigation, with ownership, receiving capacity and the consent position.
5. The undertaker's written response on sewer capacity and connection point.
6. The agreed discharge rate with its derivation.
7. Storage sizing against that rate, with the critical duration identified.
8. The exceedance and maintenance position.

Items two and five are the ones most often missing, and they are the two the reviewer looks for first. Where testing has not been done because access is impossible before demolition, say so and propose a condition rather than leaving a gap.

When a council refuses the justification

A refusal of the justification is normally a request for evidence rather than a policy objection, so the response is usually testing rather than argument. Where the authority insists on infiltration that testing shows will not work, provide the rate, the groundwater record and a statement from the engineer, and ask for the specific technical basis of the objection.

Where the dispute is about the discharge rate rather than the destination, the usual landing point is a lower rate with more storage, and it is worth knowing early what that does to the layout. Drainage is engineering work, so councils normally expect a suitably qualified author for the strategy and the calculations.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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