Section 73 Variations: Changing Flood Risk and Drainage Conditions

Planning permissions are not always implemented exactly as approved. Layouts evolve during detailed design, site conditions turn out to be different from expectations, policy changes, and commercial pressures force alterations. When the changes affect flood risk or drainage conditions, the mechanism for varying those conditions is typically a Section 73 application under the Town and Country Planning Act 1990.

Section 73 is a powerful tool, but it carries risks that are routinely underestimated. A S73 application does not simply change a condition — it creates an entirely new planning permission. That distinction has profound implications, particularly for flood risk and drainage, where the policy and data landscape can shift significantly between the original permission and the S73 determination.

This article explains when S73 is appropriate for flood and drainage changes, what can and cannot be varied, and how to manage the process to avoid unintended consequences.

When Is Section 73 Appropriate for Flood and Drainage Changes?

A S73 application is needed when the proposed change requires a variation or removal of a condition attached to the existing planning permission. In the context of flood risk and drainage, common triggers include:

Scheme layout changes affecting drainage. The approved site layout is revised — perhaps to accommodate a different housing mix, to respond to ground conditions, or to address viability issues. The revised layout changes the impermeable area distribution, the drainage network routing, or the location of SuDS features. The drainage condition on the original permission was approved on the basis of the original layout, so the condition must be varied to reflect the new layout.

Updated climate change allowances. The Environment Agency periodically updates its climate change allowances for flood risk and drainage. If the original flood risk assessment used allowances that have since been superseded, and the conditions reference specific flood levels or freeboard requirements based on those allowances, a S73 may be needed to update the conditions to reflect the current position.

New flood modelling data. The EA may have updated its flood models or Flood Map for Planning since the original permission was granted. If the new data shows a materially different flood risk from that assessed in the original FRA, conditions relating to finished floor levels, compensatory storage, or safe access may need to be varied.

Revised SuDS scheme. During detailed design or condition discharge, it may become apparent that the approved SuDS scheme is not feasible as designed — perhaps because infiltration testing has returned different results from those assumed, or because the adopted highway layout precludes permeable paving. If the change is fundamental enough that it cannot be accommodated within the existing condition wording, a S73 is needed.

Changes to finished floor levels. If the approved conditions specify minimum finished floor levels and the proposed development requires different levels (whether higher or lower), this is a change to a condition that requires S73.

What Can and Cannot Be Varied Under Section 73

Section 73 allows the LPA to grant planning permission for the development of land without complying with conditions previously attached, or subject to different conditions. However, it cannot be used to make fundamental changes to the nature of the development itself.

What Can Be Varied

• Conditions specifying drainage design parameters (discharge rates, attenuation volumes, SuDS types)
• Conditions specifying finished floor levels or freeboard requirements
• Conditions requiring specific flood mitigation measures (compensatory storage volumes, flood resilience measures)
• Conditions relating to the timing or phasing of drainage works
• Conditions referencing specific approved plans or documents (e.g., “the development shall be carried out in accordance with the approved FRA ref. XYZ dated…”)

What Cannot Be Varied

• The fundamental description of the development (e.g., changing from residential to industrial)
• The principle of development itself (if the original permission was granted on the basis that the site could be developed, S73 cannot revisit that principle — though see the caveats below about reassessment)
• Conditions imposed by statute rather than by the LPA (e.g., time limits on outline permissions)

The boundary between what is a minor variation and what constitutes a fundamental change is not always clear. If in doubt, discuss the proposed change with the LPA’s planning officer before submitting the S73 application.

Section 96A: The Non-Material Amendment Alternative

Before reaching for S73, consider whether the change is so minor that it can be dealt with under Section 96A of the TCPA 1990 — a non-material amendment (NMA).

An NMA is a simpler, faster, and cheaper process. There is no formal re-consultation with statutory consultees, no public notification, and the fee is lower (currently GBP 43 for householder applications and GBP 258 for other applications in England). The LPA simply decides whether the proposed change is non-material.

For flood and drainage purposes, an NMA might be appropriate for:

• Minor corrections to approved drawings (e.g., correcting a pipe diameter or manhole location)
• Small adjustments to finished floor levels (e.g., raising a level by 50mm to accommodate construction tolerances)
• Updating a document reference (e.g., substituting a revised FRA that reaches the same conclusions)

An NMA is unlikely to be appropriate for:

• Changes that alter the drainage strategy fundamentally (e.g., switching from infiltration to attenuation)
• Changes to flood mitigation that affect flood risk to third parties (e.g., reducing compensatory storage)
• Any change that requires re-consultation with the EA or LLFA

The test is whether the change is material to the planning decision. If the EA or LLFA would need to comment on the change, it is almost certainly material and should go through S73, not S96A.

Re-Consultation with the EA and LLFA

When a S73 application is submitted that affects flood risk or drainage conditions, the LPA will re-consult the EA and LLFA. This is a formal consultation, and the consultees will review the proposed changes against current policy and guidance — not the policy that was in place when the original permission was granted.

This re-consultation is where the risks of S73 become apparent. The EA and LLFA will assess the proposed changes on their merits, and they are not bound by their previous consultation responses. If circumstances have changed — new flood data, updated climate change allowances, revised local policy — they may raise concerns that did not exist at the time of the original permission.

What the EA Will Consider

• Whether the proposed changes are consistent with the NPPF and current Planning Practice Guidance on flood risk
• Whether the site’s flood risk has changed since the original assessment (e.g., due to Flood Map for Planning updates)
• Whether the proposed mitigation remains adequate in light of current climate change allowances
• Whether the development, as varied, would be safe for its lifetime

What the LLFA Will Consider

• Whether the revised drainage strategy complies with their current guidance (which may have changed since the original permission)
• Whether the SuDS hierarchy has been properly applied
• Whether the revised attenuation and discharge arrangements are adequate
• Whether the management and maintenance arrangements remain satisfactory

The Risk of Reassessment Against Current Policy

This is the single most important risk of S73 for flood-constrained sites, and it is frequently underestimated.

When the LPA determines a S73 application, it is granting a new planning permission. Although the principle of development has been established by the original permission, the LPA is entitled to consider the S73 against current development plan policy and material considerations — not just the specific condition being varied.

In practice, this means:

Changes to the NPPF. If the NPPF has been revised since the original permission (as it was in December 2024, with significant changes to flood risk policy), the S73 will be assessed against the revised NPPF. A development that passed the Sequential and Exception Tests under the previous NPPF may face greater scrutiny under the current version.

Updated flood maps. If the EA has updated the Flood Map for Planning since the original permission, the S73 will be assessed against the updated maps. A site that was shown as Flood Zone 2 on the original maps but is now shown as Flood Zone 3 faces a fundamentally different policy test.

New EA guidance. Updated climate change allowances, revised standing advice, or new modelling data may all be material considerations in the S73 determination.

Local plan changes. If the local plan has been updated or a new strategic flood risk assessment has been adopted since the original permission, these are material considerations.

This does not mean that S73 will be refused simply because the policy landscape has changed. But it does mean that the applicant must be prepared to demonstrate that the development, as varied, complies with current policy — or that the material considerations weigh in favour of approval despite any policy conflicts.

The Timing Issue: A New Permission with New Consequences

Because S73 creates a new planning permission, it also creates new planning consequences:

New conditions. The LPA can attach new conditions to the S73 permission that were not on the original permission. If the policy or guidance has changed, the LPA may impose more onerous conditions on drainage, flood resilience, or monitoring.

New CIL liability. The Community Infrastructure Levy is calculated at the date of commencement. If the S73 permission has a different commencement date from the original (which it will, because it is a new permission), the CIL liability may be recalculated. If CIL rates have increased, this can be a significant additional cost.

New time limit. The S73 permission has its own time limit for commencement, which runs from the date of the S73 decision — not from the date of the original permission.

Section 73 and Nutrient Neutrality

One of the most significant S73 risks to emerge in recent years relates to nutrient neutrality. Under the Conservation of Habitats and Species Regulations 2017, development that could increase nutrient loading to affected waterbodies (primarily those draining to designated habitats in catchments identified by Natural England) requires a Habitats Regulations Assessment (HRA) demonstrating nutrient neutrality.

The nutrient neutrality issue has affected large parts of England, particularly in areas draining to the Solent, Somerset Levels, Teesside, and Norfolk Broads catchments.

The S73 risk arises where:

• The original permission was granted before the nutrient neutrality requirement was identified (i.e., before Natural England’s advice was issued for the relevant catchment)
• The applicant submits a S73 to vary a condition (perhaps entirely unrelated to nutrients — for example, a change to the drainage layout)
• The LPA, in determining the S73, is obliged to carry out a new HRA because the S73 creates a new permission
• The new HRA must assess nutrient neutrality, and if the development cannot demonstrate nutrient neutrality, the S73 must be refused

This has caused significant problems for developers who held valid permissions but triggered nutrient neutrality requirements by submitting what they expected to be a routine S73. As our Hampshire nutrient neutrality case study demonstrates, the technical evidence required to resolve nutrient neutrality is substantial and must be prepared before the S73 is submitted.

The practical advice is straightforward: before submitting a S73 for a site in a nutrient neutrality catchment, check whether the S73 will trigger a new HRA and, if so, prepare the nutrient neutrality evidence in advance.

Practical Tips for Managing the S73 Process

1. Assess Whether S73 Is the Right Route

Before submitting, consider whether the change can be achieved through S96A (non-material amendment), condition discharge, or a revised submission under an existing condition. S73 should be the route of last resort, not the default.

2. Engage the LPA Early

Discuss the proposed S73 with the LPA’s planning officer and, if possible, the EA and LLFA case officers. Understand their likely position before committing to the application.

3. Prepare for Re-Consultation

Assume that the EA and LLFA will re-assess the flood risk and drainage case from first principles. Prepare updated technical evidence — an addendum to the FRA, updated drainage calculations, revised SuDS details — that addresses current policy and guidance, not just the specific condition being varied.

4. Assess the Policy Landscape

Review the current NPPF, PPG, EA guidance, and local plan policy. If the policy has changed since the original permission, assess the implications for the S73 and prepare your case accordingly. If there are potential conflicts, address them proactively in the supporting statement.

5. Check for Nutrient Neutrality and Other HRA Triggers

If the site is in a nutrient neutrality catchment, or in any location where HRA issues could arise, assess the implications before submitting. Prepare nutrient neutrality or water neutrality evidence if required.

6. Consider the Fresh Application Alternative

In some cases, a fresh full planning application may be preferable to S73. A fresh application gives you a clean slate — new conditions drafted to reflect the current scheme, no risk of inconsistency with original conditions, and a clearer planning history. The fee is higher and the process longer, but for significant changes on flood-constrained sites, it may be the safer route.

How Aegaea Can Help

Aegaea handles the technical evidence for S73 applications involving flood risk and drainage. We prepare FRA addendums, updated drainage strategies, revised SuDS designs, and nutrient neutrality assessments to support S73 submissions. We manage the re-consultation process with the EA and LLFA, ensuring that the technical case is robust before it reaches their desk.

If you need to vary flood risk or drainage conditions on an existing permission and want to understand the risks and options, contact us to discuss your project.