Evidencing the drainage hierarchy decision

Lead local flood authorities reject more drainage strategies for weak hierarchy justification than for anything else. The surface water drainage hierarchy evidence they want is not a statement that infiltration is unsuitable; it is the test results, the correspondence and the levels that show why each higher option was rejected before you landed on the one you are proposing. Part H of the Building Regulations and national drainage policy both set the order. The order is easy. Proving you worked down it is the part schemes skip.

What the hierarchy requires in order

Surface water should be discharged, in order of preference, into the ground by infiltration, then to a surface water body such as a watercourse, then to a surface water sewer, highway drain or other separate drainage system, and only then to a combined sewer.

Each step down has to be justified by evidence that the step above is not reasonably practicable. That phrasing does some work: not reasonably practicable is not the same as inconvenient, more expensive or outside the preferred layout.

A Drainage Strategy should therefore have a section that walks the hierarchy explicitly, option by option, with the evidence attached as appendices rather than summarised in a sentence. Reviewers read that section first.

One procedural note. Schedule 3 to the Flood and Water Management Act 2010, which would introduce a separate SuDS approval regime, has not been brought into force in England as at October 2026, so the lead local flood authority acts as a statutory consultee on major applications rather than as an approving body. Check the current position, because the question has been under review for some time.

Evidence that infiltration was properly tested

Infiltration is the top of the hierarchy and the option most often dismissed without testing. A sentence saying the site is underlain by clay is not evidence.

What a reviewer expects to see: trial pits or boreholes at the proposed locations and depths of the infiltration features, infiltration tests carried out to the recognised soakaway design method with the test procedure described, at least three test cycles per location with the results tabulated, the calculated infiltration rate, groundwater levels recorded during and after the works with the monitoring period stated, and a plan showing where the tests were done against the proposed drainage layout.

Then the constraints that rule infiltration out even where the ground would take water: a shallow water table with insufficient unsaturated depth beneath the base of the feature, known or suspected contamination where infiltration would mobilise it, a Source Protection Zone or other groundwater sensitivity, soluble geology and dissolution features, made ground of unknown composition, slope stability concerns, and proximity to foundations or basements.

Where infiltration works in part of the site and not elsewhere, say so and use it where it works. A partial infiltration solution with an attenuated overflow is a stronger answer than a blanket rejection.

Note: a test done in August on a dry summer and reported without groundwater monitoring will be queried. Seasonal high groundwater is the question, and the answer takes a monitoring period rather than a single visit.

Watercourse discharge and riparian ownership

Second in the hierarchy, and the step most often blocked by land rather than by hydraulics.

The practical questions are who owns the bank, whether you have a right to discharge, and whether the consent exists. Riparian ownership attaches to the land, so discharging into a watercourse you do not own needs the owner's agreement and usually an easement. Crossing third party land to reach it needs the same.

Consents are separate from planning. Works to or discharge into an ordinary watercourse need land drainage consent from the lead local flood authority, and works affecting a main river need a flood risk activity permit from the Environment Agency. The route and the evidence for the first of those is set out in land drainage consent for works to an ordinary watercourse.

Hydraulically, record the watercourse bed and bank levels, the tailwater level during the design event, whether the outfall would be drowned, and the capacity of the receiving channel. An outfall that works in dry weather and surcharges at the design event has not been assessed, and that interaction belongs in the Flood Risk Assessment as well as in the drainage strategy.

Where you reject this option, the reason has to be one of these: no watercourse within a reasonable distance, no right of discharge obtainable, insufficient capacity in the receiving channel, or levels that make gravity discharge impossible. Say which, and attach the evidence.

Approaching the sewerage undertaker and recording the answer

Third in the hierarchy, and the step where the paperwork matters most, because the evidence is a letter rather than a measurement.

Make a pre-development enquiry to the sewerage undertaker setting out the site, the proposed discharge rate and the proposed point of connection, and ask specifically whether there is a separate surface water sewer available and what rate it will accept. Section 106 of the Water Industry Act 1991 gives a right to communicate with a public sewer, and the undertaker can refuse where the mode of construction or condition of the drain would be prejudicial to its sewerage system, so the answer is not automatic.

Keep the full exchange: the enquiry with its date and reference, the undertaker's written response, any capacity assessment supplied, any conditions attached to the acceptance, and any restriction on rate. A reviewer will take a dated written response from the undertaker at face value. A note saying the undertaker was contacted will not do.

Where the response imposes a discharge rate, that rate becomes a design input and the attenuation volume follows from it. Get the answer before sizing storage rather than after.

Combined sewers and the restrictions that apply

A combined sewer is the bottom of the hierarchy and undertakers increasingly resist new surface water flows into them, because the flows end up at a treatment works or at a storm overflow.

Where a combined connection is the only option, expect a tight rate restriction, expect to be asked to demonstrate betterment on any existing connected area, and expect a requirement to remove surface water from the combined system elsewhere on the site as part of the scheme. Redevelopment sites with existing combined connections are the case where betterment is most achievable and most expected.

Document the position the same way: the undertaker's written confirmation that no separate surface water sewer is available, the rate it will accept, and the calculation showing the existing and proposed connected areas and rates.

Documenting the rejected options

Give the reviewer a single table and then the appendices. The table is what gets read.

For each step: the option, whether it was tested or investigated, what the result was, the reason for rejection, and a pointer to the appendix holding the evidence. One line per option, no narrative.

What makes the table credible rather than decorative is specificity. "Infiltration not feasible due to ground conditions" is a conclusion. "Infiltration rate measured at three locations to the recognised method, with groundwater recorded above the proposed base level at two of them, see Appendix C" is evidence. Use your own measured figures; do not borrow them.

Where an option was rejected on cost or on layout rather than on practicability, say that honestly. It may still be accepted, and it is far better than a reason that does not survive a question.

How authorities check the reasoning

Reviewers work through a predictable sequence, so test the strategy against it before submission.

1. Does the document address the hierarchy explicitly, in order, with a conclusion for each step.
2. Are the infiltration test results present, with locations, dates, method and groundwater levels, rather than asserted.
3. Is there dated written correspondence from the sewerage undertaker, with a rate.
4. Do the discharge rate, the attenuation volume and the flow control specification agree with each other across the drawings and the calculations.
5. Are the design events and the climate change allowance stated, and is the allowance the current one.
6. Is the maintenance regime set out with a named responsible party.
7. Does the surface water position in the drainage strategy match the one in the flood risk assessment, including the way surface water risk is described, which is where reading the surface water flood risk map without overstating it matters.

Where there is no mains foul connection either, the same discipline of recording what was investigated applies, and foul drainage with no mains sewer sets out what to show.

Drainage design is work councils expect from a suitably qualified engineer, and professional review before submission is strongly advised. PlanScope can draft the strategy narrative, the hierarchy justification structure and the maintenance schedule, and the testing, the correspondence and the hydraulic calculations remain the engineer's.

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