The Drainage Hierarchy: Why Infiltration Testing Comes First

Every drainage strategy for a new development in England and Wales must follow the drainage hierarchy. It is a simple concept — a priority order for how surface water runoff should be managed — but it is one of the most common reasons for drainage strategies being rejected by Lead Local Flood Authorities (LLFAs). The issue is rarely that developers have not heard of the hierarchy. It is that they have not provided sufficient evidence to demonstrate they have worked through it properly.

At the heart of the hierarchy sits infiltration testing. Without it, you cannot demonstrate that you have followed the hierarchy at all. This article explains the drainage hierarchy, why LLFAs are so insistent on evidence, and how to approach the process correctly from the outset.

What Is the Drainage Hierarchy?

The drainage hierarchy is a priority order for surface water disposal, established in Planning Practice Guidance (PPG) and reinforced by Building Regulations Part H and the Non-Statutory Technical Standards for Sustainable Drainage Systems (SuDS). It applies to all new developments and requires that the developer demonstrate, through evidence, that they have considered each option in order and have a valid reason for discounting any option before moving to the next.

The hierarchy is as follows:

Priority 1: Discharge to Ground (Infiltration)

The preferred option is to discharge surface water to the ground through infiltration — soakaways, infiltration basins, permeable paving, or other features that allow water to soak into the soil. Infiltration mimics the natural water cycle most closely, recharges groundwater, and avoids loading the sewer network or watercourses with additional flow.

Infiltration is the preferred option because it deals with surface water at or very near its source, avoiding the need for downstream infrastructure entirely. A well-designed infiltration system requires no outfall, no connection to the sewer network, and no consent from a third party. It is the simplest, most sustainable, and often the cheapest option — provided the ground conditions allow it.

Priority 2: Discharge to a Surface Water Body (Watercourse)

If infiltration is not feasible — because the soil has insufficient permeability, the groundwater table is too high, or the site is on contaminated land where infiltration could mobilise pollutants — the next option is to discharge surface water to a watercourse (river, stream, ditch, or lake).

Discharge to a watercourse requires the consent of the watercourse owner (the riparian owner, the Environment Agency, or the Internal Drainage Board, depending on the designation) and must be at a rate that does not increase flood risk downstream. For greenfield sites, this means discharging at the greenfield runoff rate; for brownfield sites, the LLFA will typically require a reduction in discharge rates compared to the existing situation.

Priority 3: Discharge to a Surface Water Sewer

If neither infiltration nor discharge to a watercourse is feasible, the next option is to connect to a surface water sewer. This requires the consent of the sewerage undertaker (the water company) and, again, must be at an attenuated rate that does not exceed the capacity of the receiving sewer.

Connection to a surface water sewer is less desirable than infiltration or watercourse discharge because it adds flow to the sewer network, which may already be at or near capacity. Many water companies are resistant to accepting additional surface water connections, and the consent process can be protracted.

Priority 4: Discharge to a Combined Sewer

The least preferred option is to discharge surface water to a combined sewer — a sewer that carries both foul water and surface water. Combined sewers are a legacy of Victorian-era drainage infrastructure, and they are a primary cause of combined sewer overflows (CSOs), which discharge untreated sewage into rivers and coastal waters during storm events.

Connecting new surface water runoff to a combined sewer exacerbates this problem and is actively discouraged by LLFAs, water companies, and regulators. It should only be proposed where all other options have been conclusively ruled out and the water company has confirmed that capacity exists.

Why LLFAs Insist on Evidence

The drainage hierarchy is not a menu from which developers can choose their preferred option. It is a sequential test: you must start at the top and work down, and you must provide evidence at each stage to justify moving to the next option. LLFAs are rigorous about this because the hierarchy exists for sound technical and environmental reasons, and without enforcement, most developers would default to the easiest option (a sewer connection) rather than the best option (infiltration).

The evidence required at each stage is as follows:

Evidence for Ruling Out Infiltration

To demonstrate that infiltration is not feasible, you must provide BRE365 infiltration test results showing that the soil infiltration rate is too low to support an infiltration system of reasonable size. A desk-based assertion that “the site is on clay” is not sufficient — the LLFA will require test data.

The BRE365 test procedure — three fills of a trial pit at the proposed soakaway depth, with the infiltration rate calculated from the later fills — is the accepted standard. The test must be carried out at the proposed soakaway location and depth, not at an arbitrary point on the site. If the results show an infiltration rate below approximately 1 x 10^-6 m/s, infiltration is generally not viable and the LLFA will accept this as justification for moving to the next option in the hierarchy.

If the infiltration rate is marginal — between 1 x 10^-6 and 1 x 10^-5 m/s — the LLFA may ask whether partial infiltration is feasible: using infiltration features to manage a proportion of the runoff, with the remainder discharged to a watercourse or sewer. This hybrid approach is increasingly common and can be a pragmatic solution for sites with variable ground conditions.

Evidence for Ruling Out Watercourse Discharge

To demonstrate that discharge to a watercourse is not feasible, you must show that there is no watercourse within a reasonable distance of the site, or that the watercourse owner will not grant consent, or that the discharge would have an unacceptable impact on flood risk or water quality downstream.

“Reasonable distance” is not defined in guidance and is a matter of engineering judgement. A watercourse 50 metres from the site boundary is clearly within reach; a watercourse 500 metres away, requiring a new pipeline across third-party land, may not be. The LLFA will expect you to have identified all watercourses in the vicinity of the site and to have considered whether a connection is practicable.

Evidence for Ruling Out Surface Water Sewer Discharge

To demonstrate that a surface water sewer connection is not feasible, you must show that no surface water sewer exists within a reasonable distance, or that the sewerage undertaker will not grant consent, or that the sewer has no available capacity. A pre-development enquiry to the water company, confirming their position on accepting surface water connections, is the standard evidence.

Why Infiltration Testing Is the Critical First Step

Infiltration testing sits at the top of the evidence chain because it determines the entire shape of the drainage strategy. If infiltration works, the drainage design can be simpler, cheaper, and more sustainable. If it does not work, the developer must pursue alternatives that involve more infrastructure, more consents, and more cost.

Critically, infiltration testing must be carried out early — before the drainage strategy is designed, not after. We regularly see developers who have designed an entire drainage layout, with soakaways sized and located on the masterplan, before carrying out any infiltration testing. When the test results show that the ground cannot accept water, the entire drainage strategy must be redesigned.

Equally common is the developer who has designed a drainage strategy based on discharge to a sewer, without carrying out infiltration testing first. When the LLFA reviews the strategy, the first question is invariably: “Where are the infiltration test results?” Without them, the strategy is rejected, and the developer must carry out testing before resubmitting — adding weeks or months to the programme.

The message is straightforward: carry out BRE365 testing at the earliest opportunity, ideally during the site investigation phase, before any drainage design work begins. The results will determine which option in the hierarchy is appropriate and will inform the drainage design from the outset.

Practical Application

Greenfield Sites

On greenfield sites, the LLFA expectation is usually that surface water will be managed through infiltration or, failing that, discharge to a watercourse at the greenfield runoff rate. The drainage strategy should demonstrate how the development will mimic the pre-development drainage regime as closely as possible, using SuDS features throughout the management train.

BRE365 testing on greenfield sites should be carried out at multiple locations across the site, particularly where the geology is expected to vary (for example, across the boundary between different soil types or where the depth to bedrock changes). The results will determine which areas of the site are suitable for infiltration features and which are not.

Brownfield Sites

On brownfield sites, the LLFA expectation is that the development will reduce surface water runoff compared to the existing situation. This is a significant opportunity: most brownfield sites have large areas of impermeable surface (roofs, hardstanding, car parks) that discharge directly to combined sewers at unrestricted rates. Replacing this with an attenuated, SuDS-based drainage system delivers a genuine betterment.

Infiltration testing on brownfield sites requires careful consideration of ground contamination. If the Phase 1 desk study or Phase 2 investigation identifies contamination in the soil or groundwater, infiltration may not be appropriate because it could mobilise contaminants and create new pollution pathways. In these cases, the drainage hierarchy allows infiltration to be ruled out on contamination grounds, but the evidence must be clearly presented.

Constrained Urban Sites

Urban infill sites often present the most challenging drainage hierarchy problem. There may be no watercourse nearby, the local surface water sewer may be at capacity, and the ground conditions may not support infiltration. In these cases, the drainage strategy must work creatively within the hierarchy, potentially combining partial infiltration (where ground conditions allow), above-ground SuDS features for treatment and attenuation, and a restricted discharge to the sewer network.

The LLFA will expect to see that all options have been genuinely explored and that the proposed solution is the best achievable within the site constraints. Simply defaulting to a sewer connection because the site is in an urban area is not acceptable.

Common Reasons for LLFA Rejection

Based on our experience working with LLFAs across England and Scotland, these are the most common reasons for drainage strategies being rejected at the hierarchy stage:

1. No infiltration test results provided. The most common reason. The developer has proposed a sewer discharge without demonstrating that infiltration is not feasible.

2. Infiltration test results at the wrong location or depth. The test was carried out at a location that does not correspond to the proposed soakaway, or at a depth that does not match the design.

3. No consideration of watercourse discharge. The developer has jumped from infiltration directly to a sewer connection without considering whether a watercourse discharge is feasible.

4. Insufficient justification for ruling out options. A single sentence stating “infiltration is not feasible due to clay soils” is not adequate justification. The LLFA requires test data.

5. No SuDS management train. The drainage strategy proposes a pipe-and-tank system with no above-ground SuDS features. Even where infiltration is not viable, the LLFA expects to see SuDS features for treatment and amenity.

Getting It Right from the Start

The drainage hierarchy is not an obstacle to development — it is a framework for good drainage design. Working through it properly, with the right evidence at each stage, produces drainage strategies that are more sustainable, more resilient, and more likely to be approved by the LLFA without protracted negotiation.

The critical first step is always infiltration testing. Commission it early, carry it out at the right locations and depths, and let the results drive the drainage design rather than the other way around.

If you need help navigating the drainage hierarchy for your site, or if you need BRE365 testing to support your drainage strategy, get in touch. We work with LLFAs across the country and we know what they expect to see. Getting the hierarchy evidence right at the outset is one of the most effective ways to keep your surface water drainage strategy on track and avoid costly delays.