Proving infiltration is unfeasible before discharging
Surface water strategies fail because the applicant jumped to a sewer without showing why infiltration could not work. Here is the evidence that discounts each rung properly.
Drainage officers reject surface water strategies for one reason more than any other, which is that the applicant jumped straight to a sewer connection without showing why infiltration could not work. The discharge hierarchy evidence they want is specific. Measured infiltration rates from trial pits in the right positions, a groundwater level, a ground conditions review, and a reasoned statement of why each higher option in the order was discounted. A sentence saying the ground is clay is not evidence, and neither is a drainage engineer's professional opinion unsupported by a test result.
The order you have to work down
The hierarchy comes from Approved Document H of the Building Regulations and is repeated in national planning guidance and in the non-statutory technical standards for sustainable drainage. Rainwater should discharge to the ground by infiltration first, to a surface water body second, and to a sewer last.
Many authorities add a step at the top. Rainwater harvesting and reuse, then infiltration, then watercourse, then surface water sewer, then as an absolute last resort a combined sewer, which most undertakers now refuse outright for new surface water.
Working down the order is a sequential exercise and you have to show your working at each rung. An officer who cannot see why option one was discounted will not look at option three.
| Position in the order | Evidence normally expected |
|---|---|
| Infiltration to ground | Trial pit infiltration tests, groundwater levels, contamination and stability review |
| Discharge to watercourse | Riparian rights, outfall design, consent from the drainage authority, restricted discharge rate |
| Surface water sewer | Written capacity response from the undertaker, restricted rate, attenuation volume |
| Combined sewer | Refusal of the above in writing, and usually a betterment on existing rates |
What counts as evidence that infiltration fails
Infiltration testing follows the method in the Building Research Establishment digest on soakaway design, and the drainage officer will check four things about how it was done.
- Position. Tests in the footprint of the proposed soakaway, at the proposed depth, not in a convenient corner of the site at 600mm.
- Repetition. Three successive test fills in each pit, with the rate taken from the fall between the specified proportions of the effective storage depth.
- Number. Enough pits to cover the variation across the site, usually at least one per proposed infiltration feature.
- Reporting. Pit logs with a soil description, the water levels and times recorded, and the calculated rate stated with its units.
A rate low enough to make a soakaway impractically large is a legitimate reason to move down the order, and the way to show it is to size the feature on the measured rate and state the footprint and depth it would need. "The soakaway would be 180 cubic metres and occupy the whole of the rear garden" is an argument. "Infiltration is not feasible" is not.
Winter testing is more persuasive than summer testing on marginal sites, because a desiccated clay can give a flattering result in August that the same pit will not repeat in February. Say when the tests were carried out.
Groundwater, contamination and ground stability
Even a good infiltration rate can be unusable. Four conditions commonly rule it out, and each needs its own piece of evidence.
Groundwater level. A soakaway needs unsaturated ground beneath its base, and the usual expectation is a clear separation between the base of the feature and the highest recorded water table. One dip in a trial pit is weak evidence. Monitoring over a wet period, or a reasoned estimate from the geological memoir and local borehole records held by the British Geological Survey, carries more weight.
Contamination. Infiltrating water through contaminated ground mobilises contaminants towards the water table, and the Environment Agency objects to it. Where a Phase 1 desk study identifies a potentially contaminative historic use, or where the site lies in a Source Protection Zone, say so and cross-refer to the land contamination work. A suitably qualified and experienced person must prepare contamination reports in accordance with the relevant British Standard, and an AI-assisted draft is a starting point for that professional rather than a substitute for them.
Ground stability. Made ground, soluble strata, shallow mine workings, shrinkable clays close to foundations and sloping sites all argue against concentrating water into the ground. The relevant evidence is the geotechnical report and, in coalfield areas, the mining report.
Space and proximity. Soakaways are normally kept a stated minimum distance from building foundations, and on a tight plot that alone can remove the option. Show the constraint on a plan rather than describing it.
A Drainage Strategy should carry these four tests as a short section with a conclusion against each, because that is the structure a lead local flood authority consultation response follows.
Moving to a watercourse and the consents needed
A watercourse discharge is the preferred fallback and it brings three separate questions.
The first is whether you have a right to discharge. Riparian ownership of the bank, or an easement across land to reach it, has to exist before the strategy is credible. Showing an outfall on third party land with no evidence of a right invites the officer to discount the whole option.
The second is the rate. Discharge is normally restricted to the greenfield runoff rate for the site, calculated by an accepted method, with attenuation storage provided on site for the difference up to the one in 100 year event plus a climate change allowance. The allowances are revised periodically, so take them from the current published tables rather than from an earlier report.
The third is consent. Works to an ordinary watercourse, including a new outfall, need land drainage consent from the lead local flood authority under the Land Drainage Act 1991, and land drainage consent for works to an ordinary watercourse runs on its own timetable alongside the planning application. Where the receiving water is a main river, the Environment Agency permit route applies instead.
Outfall design gets overlooked. Headwall, scour protection, non-return flap where the receiving level can back up, and a maintenance access point. A drainage officer who sees those details treats the rest of the strategy more generously.
Connecting to a sewer as the last resort
The right to connect to a public sewer comes from the Water Industry Act 1991, and the undertaker can refuse on grounds relating to the mode of construction or the condition of the connection. Capacity is handled through the undertaker's own assessment rather than through a flat right of refusal, which is why the written response matters so much.
What the drainage officer wants to see is the undertaker's response to a pre-development enquiry, naming the manhole or sewer to be used, confirming or restricting the discharge rate, and stating any capacity condition. A generic statement that the site will connect to the public sewer carries no weight at all.
Expect the restricted rate to be low. Several undertakers now apply a limit well below the greenfield rate for brownfield redevelopments as a betterment requirement, which increases the attenuation volume and can change the layout, so get the answer before the site plan is fixed.
Pre-development enquiries and when to send them
Three enquiries decide the drainage strategy, and all three take weeks.
Send the sewerage undertaker a pre-development enquiry with a site plan, the proposed impermeable area and the proposed discharge rate. Send the lead local flood authority a pre-application question on its expectations for infiltration testing and discharge rates, because those vary between authorities more than any other drainage requirement. And order the ground investigation, including infiltration testing, early enough that the results inform the layout rather than contradicting it.
The sequencing error that costs most is designing the scheme, then testing the ground, then discovering that the attenuation volume will not fit under the parking court.
Assembling the evidence pack
For a minor residential scheme, a sound pack is the infiltration test report with pit logs and locations, a groundwater statement, a geotechnical and stability summary, a contamination screening conclusion, the greenfield runoff calculation with its method named, the proposed discharge rate and attenuation volume, the undertaker's pre-development enquiry response, the drainage authority's consent position, drainage layout drawings and a maintenance plan.
A Flood Risk Assessment is a separate document where the site falls within Flood Zone 2 or 3 or covers one hectare or more, and it should cross-refer to the drainage strategy rather than repeat it. Foul drainage runs on a different track again, and sites with no mains connection need what to show for foul drainage handled as its own piece of work.
Enter the postcode on PlanScope to see which drainage and flood reports a site is likely to need before you commission the ground investigation.
This is general guidance, not legal advice. Check the position with your local planning authority, the lead local flood 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.
Frequently asked questions
What is the surface water discharge hierarchy?
Infiltration to ground first, discharge to a surface water body second, a surface water sewer third and a combined sewer as an absolute last resort. The order comes from Approved Document H of the Building Regulations and is repeated in national planning guidance and the non-statutory technical standards for sustainable drainage.
Is a clay soil enough to rule out infiltration?
No. The authority wants a measured rate from trial pits in the footprint of the proposed soakaway at the proposed depth, with three successive test fills in each pit, and then the feature sized on that rate. Stating the footprint and depth a compliant soakaway would need is the argument that works.
Why does contamination rule out a soakaway?
Infiltrating water through contaminated ground drives contaminants downwards towards groundwater, which the Environment Agency objects to. Where a Phase 1 desk study flags a contaminative historic use or the site sits in a Source Protection Zone, the strategy should say so and cross-refer to the contamination work by a suitably qualified person.
What consents does a new outfall to a watercourse need?
Works to an ordinary watercourse, including a new outfall, need land drainage consent from the lead local flood authority under the Land Drainage Act 1991. Where the receiving water is a main river, an Environment Agency permit applies instead. You also need a riparian right or an easement to reach the bank.
When should I send a pre-development enquiry to the water company?
Before the site layout is fixed. The undertaker's response names the sewer to be used and sets the permitted discharge rate, and several undertakers now require a rate below greenfield for brownfield redevelopment. That rate drives the attenuation volume, which in turn drives where the parking and landscaping can go.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.