The SuDS Hierarchy Explained

The SuDS hierarchy is the order in which a development has to try to get rid of its surface water: reuse it first, then let it soak into the ground, then send it to a watercourse, then a surface water sewer, and only as an absolute last resort into a combined sewer. Formally it is Standard 1 of Defra's National Standards for Sustainable Drainage Systems, and in practice it decides more than almost anything else in a drainage strategy whether a Lead Local Flood Authority accepts or objects to a planning application.

This guide sets out the current, five-tier version of the SuDS hierarchy in full, how it changed when Defra rewrote the standards in June 2025, the narrower hierarchy that applies specifically to connecting to a watercourse, how it differs from the separate hierarchy that governs foul drainage, and a worked example of how a real site is expected to work through it.

Cost alone is never sufficient justification to move down the hierarchy: Defra's 2025 standards state directly that a higher-priority destination can only be ruled out with evidence it was used to the maximum extent practicable, not because a lower-priority option is cheaper.

What is the SuDS hierarchy, in order?

“SuDS hierarchy” gets used loosely for two different things in practice, and it is worth being precise about which one this article covers. There's the destination hierarchy (where the water ultimately goes) and there's the separate SuDS management train, the sequence of physical features, source control, then site control, then regional control, that water passes through on its way across a site. This article is about the destination hierarchy: the one Lead Local Flood Authorities actually test a drainage strategy against. For the management train and source control, see Unda's explainer on source control in UK drainage.

The destination hierarchy, set out in Standard 1 of the 2025 National Standards for Sustainable Drainage Systems, runs:

Where more than one of these is used on a single site, each has to be maximised in priority order before the design moves to the next. A scheme can quite legitimately combine, say, partial infiltration with an attenuated discharge to a watercourse, rather than treating the hierarchy as an all-or-nothing choice.

How the SuDS hierarchy changed in June 2025

For years, the practical source for this hierarchy was Approved Document H of the Building Regulations, which gives a much shorter three-step order: an adequate soakaway or infiltration system, then a watercourse, then a sewer. A common misconception is that Defra's original 2015 Non-Statutory Technical Standards for SuDS set out a destination hierarchy of their own. They did not. The 2015 standards dealt with peak flow, volume control and structural design, and left the destination question to Approved Document H and the CIRIA SuDS Manual.

That changed when Defra published new National Standards for Sustainable Drainage Systems on 19 June 2025 (updated 30 July 2025), replacing the 2015 standards. The five-tier version above is the current, operative hierarchy, and the change is not cosmetic: rainwater harvesting now sits above infiltration rather than being an afterthought, and every step down the list has to be justified in writing, not merely asserted. Defra frames the whole approach behind the hierarchy this way:

Discharging to a watercourse: a hierarchy within the hierarchy

Landing on “watercourse” as your destination is not the end of the decision. Standard 4 of the National Standards sets out its own, narrower order for how that connection should actually be built:

1. Use an existing or new, appropriately vegetated channel, removing any redundant existing structures where possible.
2. Failing that, reuse an existing engineered drainage outfall.
3. Only where neither of those is available, build a new engineered outfall, designed with regard to the receiving watercourse's hydromorphology and agreed with the approving body and other risk management authorities.

So even after a scheme has correctly worked its way down to a watercourse on the main SuDS hierarchy, there's a second, smaller hierarchy governing exactly how that outfall is built.

A worked example: how a site actually moves down the hierarchy

Take a hypothetical 60-dwelling scheme on a brownfield site with clay subsoil and a seasonally high water table. The designer starts at priority 1: is there a genuine non-potable demand, a landscape irrigation need, or is the site in a water-stressed area? If so, rainwater harvesting has to be considered and sized to BS EN 16941. It does not need to handle all the runoff to be required, just some of it.

Next comes BRE Digest 365 infiltration testing. On this hypothetical site, the results come back too slow across most of the clay-dominated area, so infiltration cannot be relied on there under the standards. A smaller pocket of made ground with better drainage does test favourably, though, and gets used for a partial infiltration system. That's a legitimate blend rather than a compromise. For the runoff that cannot infiltrate, the next question is whether a watercourse exists nearby and whether the relevant risk management authority will agree discharge rates and volumes in writing. If not, the design falls to a surface water sewer, subject to written confirmation from the sewerage undertaker that it actually has spare capacity. Only if that capacity genuinely is not available, and cannot practically be created, does the scheme fall back to a combined sewer, at which point the pollution risk from surcharging that sewer also has to be assessed.

A strategy that jumps straight to “connect to the combined sewer” without documenting why the higher priorities were not practicable is one of the most common reasons a Lead Local Flood Authority objects. That evidential burden sits with the applicant at every step, not only at the final destination chosen.

The SuDS hierarchy vs the foul drainage hierarchy — they are not the same

Approved Document H actually contains two separate hierarchies, and mixing them up is a genuine, recurring error. Requirement H3 governs rainwater and surface water, the SuDS hierarchy this article is about. Requirement H2 governs foul drainage, for wastewater rather than rainwater, and uses a completely different order.

A site can quite reasonably fail its surface water infiltration test and pass its foul percolation test, or the other way round. The two hierarchies solve different problems, using different tests, even though both live inside the same building regulation. For the detail on the foul side, see Unda's guide to BS 6297 infiltration testing for foul drainage fields, and for the surface water side, BRE365 infiltration testing and soakaway design.

Is the SuDS hierarchy legally binding?

Not directly, and this catches people out. The 2025 National Standards remain non-statutory. Schedule 3 of the Flood and Water Management Act 2010, which would create a statutory approval regime for SuDS, has never been commenced in England, despite a 2023 government pledge to bring mandatory SuDS in “from 2024.” The most recent parliamentary statement on the subject, in January 2026, still described it as a matter to be decided “in due course,” with no commencement date set. The full background is in Unda's guide to the Flood and Water Management Act 2010.

What does carry weight is the National Planning Policy Framework, which expects sustainable drainage on any development with drainage implications “unless there is clear evidence that this would be inappropriate,” and Defra's 2025 standards, which are what Lead Local Flood Authorities actually assess a strategy against in practice. Non-statutory does not mean optional in any way that matters to an applicant. It means the enforcement route runs through the planning system rather than a separate approval body, at least until (or unless) Schedule 3 is switched on.

How LLFAs check compliance with the SuDS hierarchy

Lead Local Flood Authorities are the statutory consultee on surface water drainage for major development, and they assess a strategy directly against the SuDS hierarchy: has infiltration genuinely been tested and ruled out before moving down the list, are the discharge rates and volumes agreed and evidenced, and is there a credible reason for using a lower-priority destination rather than a cheaper convenience. A growing number of authorities, including every London borough, now also expect a standardised SuDS proforma alongside the full strategy, a fixed-format summary cross-referenced to exactly where each figure is evidenced. It summarises the strategy; it cannot substitute for the ground investigation and testing behind it. Unda's guide to what a SuDS proforma is and when one is required and to resolving an LLFA objection both cover this in more depth.

Frequently asked questions

Getting the SuDS hierarchy right, and evidencing it properly, is one of the most common reasons a drainage strategy stalls at planning. Unda's surface water drainage strategy service handles the infiltration testing, hierarchy justification and LLFA liaison from the outset, so the strategy is right the first time it is submitted.