The drainage hierarchy and justifying a departure
Surface water must go to ground unless infiltration can be shown to fail. Here is the evidence needed to move down each step of the drainage hierarchy without a holding objection.
Surface water goes into the ground first. Only where infiltration can be shown to fail does a scheme move to a surface water body, then to a surface water sewer, then, as a last resort, to a combined sewer. Lead local flood authorities apply that order on almost every application they comment on, and the commonest reason a drainage strategy comes back with a holding objection is that the designer jumped to step three without evidencing steps one and two.
The evidence is the whole job. An assertion that infiltration is unsuitable carries no weight at all.
Where the order comes from
Two documents set it out. Requirement H3 of the Building Regulations, through Approved Document H, requires rainwater to discharge to an adequate soakaway or infiltration system, and where that is not reasonably practicable, to a watercourse, and only then to a sewer. Defra's non-statutory technical standards for sustainable drainage systems repeat the same sequence for planning purposes, splitting the sewer option into a surface water sewer, a highway drain or another drainage system, with a combined sewer last.
Schedule 3 to the Flood and Water Management Act 2010 would put SuDS approval on a statutory footing with its own approving bodies. It has not been brought fully into force in England, so the hierarchy currently reaches applications through planning conditions, the lead local flood authority's consultee role on major development, and the Building Regulations at the construction stage. Check whether that has changed, because the start date has been announced more than once.
A drainage strategy that quotes the hierarchy and then works down it, with a reason at each step, is the document the consultee is looking for. That structure is what a Drainage Strategy sets out to deliver.
Infiltration first, and proving it fails
Infiltration is ruled out for a small number of reasons, and each one has a recognised test behind it.
| Reason infiltration is rejected | Evidence expected |
|---|---|
| Soils too impermeable | Infiltration testing to BRE Digest 365 at the proposed depth and location, with results in metres per second |
| Groundwater too high | Monitored groundwater levels, ideally seasonal, showing less than the required unsaturated depth beneath the base |
| Contaminated ground | Phase 1 and, where warranted, Phase 2 findings showing a pollutant linkage that infiltration would activate |
| Source protection zone | The Environment Agency's groundwater designation and its position on infiltration in that zone |
| Ground stability | Solution features, made ground, mining legacy or shrinkable clay close to foundations |
Three points about testing. Do it at the depth the soakaway will actually sit, because a trial pit at 1 metre tells you nothing about a basin at 3 metres. Do enough of them, since a single test on a site of any size will be challenged. And report the raw data, not a conclusion, because the consultee wants to see the fall over time rather than a stated rate.
Partial infiltration is the answer more often than designers assume. A site that cannot take the whole volume may still take the first few millimetres of rainfall through permeable paving or a lined basin with an infiltration base, which reduces the discharge rate downstream and improves water quality. Rejecting infiltration entirely on a marginal permeability result is a choice a consultee may push back on.
Discharge to a surface water body
Step two is discharge to a watercourse, a lake or another surface water body. The planning question is whether there is a viable outfall, and the consenting question is who controls it.
For an ordinary watercourse, works affecting flow need consent from the lead local flood authority under section 23 of the Land Drainage Act 1991, or from the internal drainage board in its district. For a main river, the works are a flood risk activity and need an environmental permit from the Environment Agency. Neither consent is granted by the planning permission, and both take time.
Three practical obstacles come up repeatedly. Third-party land between the site and the outfall, which needs an easement negotiated before the strategy can be relied on. Outfall capacity and the condition of the receiving channel, which may already be surcharging. And the structure itself, with headwall design, scour protection and future access, covered in the note on trash screens, headwalls and maintenance access.
Discharge rates are set locally. Greenfield sites are normally restricted to the greenfield runoff rate for the relevant return periods, and brownfield sites are usually asked for a reduction against the existing rate. Many authorities apply a minimum practical discharge rate to avoid blockage in small orifices. Confirm the figures with the lead local flood authority rather than copying them from another scheme.
Surface water sewers and the right to connect
Section 106 of the Water Industry Act 1991 gives an owner a right to connect drains and sewers to a public sewer, and the undertaker can refuse where the mode of construction or condition of the drain is unsatisfactory or where the connection would be prejudicial to the sewerage system. In practice the conversation happens through a pre-development enquiry, which returns the undertaker's position on capacity and any rate restriction.
Get that enquiry in early. A response takes weeks, it often imposes a discharge rate lower than the designer assumed, and it occasionally identifies that the sewer the site plan shows as surface water is in fact combined.
Attenuation volume follows from the permitted rate. Halving the allowed discharge does not halve the storage, it roughly doubles it, and on a tight site that is the difference between a basin in the public open space and an oversized tank under the access road.
Combined sewers as the last resort
Connecting surface water to a combined sewer adds clean water to a system designed to carry foul flows to treatment, which increases spill frequency at storm overflows. Undertakers resist it, and several now refuse new surface water connections to combined systems altogether except where no alternative exists.
Where it is unavoidable, expect a tightly restricted rate, a requirement to demonstrate that every earlier step has been properly tested, and sometimes an obligation to remove an equivalent volume of existing surface water from the combined system elsewhere on the site. That last point is worth exploring on redevelopment sites, since disconnecting existing roof drainage is a real improvement and the strategy should claim credit for it.
Writing the rejection down
The hierarchy is a sequence of decisions, and each one needs a short, evidenced paragraph. Set it out in order:
- State the option being considered and the standard against which it was tested.
- Give the evidence, with the test method, date, location and result.
- State the conclusion and why the option is not reasonably practicable.
- Move to the next step.
Appendices carry the raw data: infiltration test sheets, borehole logs, groundwater monitoring records, the pre-development enquiry response, correspondence with the lead local flood authority, and any landowner consent for an outfall. A strategy that says infiltration was found to be unsuitable, with nothing behind it, will be asked for all of this anyway, three weeks into the determination period.
Where ground conditions cut across the order
Sites with shallow rock, high groundwater, deep made ground or a mining legacy make the top of the hierarchy hard and the middle unavailable. Those are the schemes where an early conversation matters, because the drainage solution dictates the levels strategy, and the levels strategy dictates the layout.
There is an honest tension here that is rarely acknowledged. The hierarchy favours infiltration for water quantity and quality reasons, while contaminated land practice and Part IIA thinking can treat infiltration as the mechanism that mobilises a contaminant. Where both apply, the resolution is usually a lined system with treatment, and the strategy should explain why rather than quietly skipping a step.
Where the site also sits in Flood Zone 2 or 3, the drainage strategy sits alongside, and must be consistent with, the Flood Risk Assessment. Inconsistent finished floor levels between the two documents are among the easiest errors for a consultee to spot, and the note on what a consultee checks in a drainage model output covers the rest of that checklist.
How the authority tests the case
Officers look for four things: that the hierarchy was worked through in order, that each rejection rests on site-specific data rather than assumption, that the discharge rate matches what the receiving system will accept in writing, and that the proposed features can be maintained by someone identifiable for the design life.
That last one sinks more schemes than people expect. A permeable surface in a shared car park with no management company, or a basin in a rear garden, will attract an objection on maintenance alone, whatever the hydraulics say.
Enter your postcode on PlanScope to see which flood and drainage documents a site is likely to need before the layout is fixed.
This is general guidance, not legal advice. Drainage design and infiltration testing should be carried out by a suitably qualified engineer, and you should check the position with your lead local flood authority before relying on any of this. 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 drainage hierarchy?
Discharge to ground by infiltration first, then to a surface water body such as a watercourse, then to a surface water sewer, highway drain or other drainage system, and only as a last resort to a combined sewer. The order appears in Approved Document H to the Building Regulations and in Defra's non-statutory technical standards for sustainable drainage systems.
What evidence shows that infiltration is not feasible?
Infiltration testing to BRE Digest 365 carried out at the depth and location of the proposed feature, with raw fall-over-time data rather than a stated rate. Add groundwater monitoring showing insufficient unsaturated depth, contamination findings, a source protection zone designation, or ground stability constraints. A single trial pit on a site of any size will be challenged.
Do I need consent to discharge into a watercourse?
Yes, separately from planning permission. Works affecting flow in an ordinary watercourse need consent under section 23 of the Land Drainage Act 1991 from the lead local flood authority or internal drainage board. Works on a main river are a flood risk activity needing an environmental permit from the Environment Agency. Both take time and neither is granted by the permission.
Can a water company refuse a surface water sewer connection?
Section 106 of the Water Industry Act 1991 gives a right to connect, but the undertaker can refuse where the mode of construction or condition of the drain is unsatisfactory or where the connection would be prejudicial to the sewerage system. Submit a pre-development enquiry early, because responses take weeks and often impose a lower rate than assumed.
Is the SuDS approving body regime in force?
Schedule 3 to the Flood and Water Management Act 2010 would create SuDS approving bodies with a statutory approval stage, but it has not been brought fully into force in England. The hierarchy currently reaches applications through planning conditions, the lead local flood authority's consultee role on major development, and the Building Regulations. Check the current position.
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.