Refused on infiltration drainage? Test the ground first

A drainage strategy that says surface water "will be managed by infiltration" without a single percolation test is one of the easier refusals to predict. The lead local flood authority reads it, cannot verify the assumption, and recommends refusal or an objection that the case officer converts into a reason. On a minor application there may be no statutory consultation at all, in which case the officer simply refuses on inadequate drainage information. Either way the fix is the same: test the ground, then design.

Infiltration is the top of the drainage hierarchy for good reasons. It keeps run-off out of sewers and watercourses and it is usually the cheapest thing to build. But the hierarchy is a preference, not a permission, and the burden sits with the applicant to show the ground will actually take water.

Why an untested infiltration assumption fails

Three things go wrong at once when a strategy assumes infiltration.

The soakaway or basin is sized on an assumed infiltration rate, so the storage volume in the drawings has no evidential basis. The discharge rate used in the attenuation calculation inherits the same error. And the finished levels, which depend on how deep the feature has to go, may not be achievable once real groundwater is known.

A consultee cannot sign that off, and an officer who grants permission on it is exposed if the scheme floods. So the response is either a refusal reason about insufficient information or, on a better day, a request to withdraw and resubmit.

Infiltration testing and the published method

The method everyone expects is the soakaway test in BRE Digest 365, with the variant tests in the SuDS Manual for larger or shallower features. In outline, a trial pit is dug to the proposed depth of the feature, filled with water, and the fall in level is timed, usually across three successive fills so the result reflects wetted ground rather than dry soil soaking up the first load.

What a reviewer looks for in the report:

• Pit locations plotted on the drainage layout, at or near the proposed features rather than in the corner of the site
• Pit depth matched to the proposed invert and base levels of the feature
• Three test cycles per pit with the recorded times, not a single average
• The soil infiltration rate in metres per second, with the factor of safety applied and stated
• Groundwater level encountered, and the date of the test

Plenty of submitted reports give a rate with no pit location and no date. That is treated as no test.

Groundwater levels and seasonal variation

A test in late August says little about February. Groundwater in much of lowland England rises through winter, and the relevant figure for soakaway design is the seasonal maximum, not what the auger found on the day.

The usual expectation is unsaturated ground beneath the base of the feature, with a margin set out in BRE Digest 365, so a shallow winter water table can rule out infiltration even where the summer rate looked respectable. Where the risk is real, monitoring over months rather than a single visit is the honest answer, and a condition requiring monitoring before works start is a reasonable thing to offer.

Where infiltration is unsuitable, and what replaces it

Infiltration is the wrong answer on several kinds of site. Made ground over an unknown fill. Land where contamination has been identified, because infiltrating water mobilises it towards groundwater. Source protection zones, where the Environment Agency takes a close interest. Steep ground, where infiltration can affect slope stability. Shrinkable clay close to foundations. And dissolution-prone ground, where concentrating water is how you make a hole.

If a Land Contamination assessment has flagged a plume or a hotspot, expect infiltration to be resisted outright, and say so in the strategy rather than leaving the consultee to find the contradiction between two of your own documents.

Discharge hierarchy and consent from the undertaker

Once infiltration is out, the order is a surface water body, then a surface water sewer, then a combined sewer as a last resort. Each step needs someone else to agree. A discharge to a watercourse may need land drainage consent from the lead local flood authority or the Environment Agency, depending on whether the watercourse is designated a main river.

A sewer connection runs through the water undertaker, and the right to connect under the Water Industry Act 1991 is not the same as a right to discharge whatever rate you like. Undertakers routinely require restriction to greenfield or near-greenfield rates and will ask for the calculation. Get that correspondence started before you resubmit, because a letter from the undertaker answers a question the council cannot answer for itself.

Lead local flood authority consultation responses

For major development the lead local flood authority is a statutory consultee on surface water drainage, a role it has held since April 2015 under the arrangements flowing from the Flood and Water Management Act 2010. Its responses tend to follow a pattern, and reading three or four recent ones for your authority before you submit is time well spent. Many publish a local guidance note setting out what they expect, including whether they will accept an assumed rate at outline stage and what they want conditioned.

For minor development there is often no statutory consultation, but many authorities still take internal drainage advice, and the validation checklist may require a drainage strategy anyway. A Flood Risk Assessment and a drainage strategy are separate documents answering separate questions, and submitting one in place of the other is a common reason for an invalid application.

Fixing the strategy before resubmission

The repair work is mostly fieldwork and arithmetic.

1. Commission infiltration testing to the published method at the proposed feature locations and depths, and record groundwater.
2. Recalculate storage with the measured rate and the stated factor of safety, for the design events and the climate change allowance the Environment Agency currently publishes.
3. If infiltration fails, work down the hierarchy and secure the receiving party's position in writing.
4. Set finished floor levels and overland flow routes for the exceedance event, and show them on a drawing.
5. Rewrite the Drainage Strategy so the design follows the evidence, with maintenance and adoption arrangements stated.
6. Address the refusal reason directly in the covering narrative rather than hoping a different officer reads it differently.

Where the refusal has already been issued and the evidence is clear, a fresh application is usually faster than an appeal, because the dispute is about information rather than principle. Appeal evidence is worth gathering when the authority has rejected a tested design on judgement, and reading how inspectors have handled comparable drainage disputes helps; there is a longer piece on using appeal decisions in an application.

One caveat on climate change allowances: they are revised, and the percentage uplift you used eighteen months ago may not be the current one. Check the published allowances for your river basin district at the point of resubmission.

PlanScope can draft a drainage strategy structured to national policy and the local plan, with a clear flag wherever a site fact such as an infiltration rate is missing, which is exactly the gap this refusal reason exposes. Enter your postcode on PlanScope to see which reports 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.