Sequential Test Accepted for London Residential Development

The Project:

A developer was preparing a planning application for a new residential dwelling on a constrained urban site in west London.

The site was suitable in principle for residential use, but it lay within Flood Zone 3, associated with the defended tidal floodplain of the River Thames. For planning purposes, this meant the proposal could not rely on design mitigation alone. It also needed a clear Sequential and Exception Test.

Residential development is classed as "more vulnerable" under national planning policy. The planning submission therefore needed to show that there were no reasonably available lower-risk sites capable of accommodating a comparable proposal, and that the development could be made safe for its lifetime.

Unda was instructed to prepare the Sequential and Exception Test evidence alongside the site’s Flood Risk Assessment for planning.

The Challenge:

The main risk was policy compliance.

The flood risk evidence showed that the site benefited from formal tidal defences and was outside the relevant modelled breach extents. That was important, but it did not remove the need for the Sequential Test. A site can be technically manageable and still need to justify why development should be permitted in a higher flood risk location.

This point has become more important since the September 2025 PPG update on the flood risk Sequential Test, which clarified proportionality, all-sources flood risk and the meaning of "reasonably available" alternatives.

The area of search also needed careful handling. If drawn too widely, the assessment risked comparing the site with alternatives that did not serve the same local housing market or development need. If drawn too narrowly, it could be rejected as self-serving.

Without a structured and proportionate case, the application risked delay, further information requests, or refusal on sequential grounds.

What We Did:

Unda prepared a Sequential and Exception Test that treated the issue as a planning judgment, not a simple mapping exercise.

We first reviewed the site’s flood risk position using the available flood risk evidence, local policy context and strategic flood risk information. The site remained within Flood Zone 3 for planning purposes, but the technical evidence showed that the main risk was residual tidal flood risk within a defended Thames floodplain, rather than direct inundation from a modelled breach event.

We then defined a proportionate area of search. The proposal was for a single residential dwelling within a dense west London housing market. It was not a strategic allocation or a major housing site. The assessment therefore focused on realistic alternatives capable of accommodating the same type and scale of development within the relevant local context.

Alternative sites were reviewed against the core Sequential Test questions: whether they were suitable for the same form of development, whether they could meet the same need, and whether they had a realistic prospect of delivery in a comparable timeframe.

For the Exception Test, we structured the evidence around the two required limbs: whether the proposal provided wider sustainability benefits, and whether it could be made safe for its lifetime without increasing flood risk elsewhere.

The safety case drew on the wider Flood Risk Assessment evidence and the proposed design response. This included safe internal refuge, flood-resilient construction, appropriate drainage design and surface water management. That broader drainage point matters in London, where the 2025-30 London Surface Water Strategy has increased the emphasis on managing rainfall within dense urban catchments.

We also considered how the case should be presented in light of recent legal and appeal commentary, including Unda’s analysis of what the Gladman judgment means for the Sequential Test. The assessment separated the location question, the site safety question and the planning balance, so the decision-maker could understand each part of the flood risk case.

The Outcome:

The Sequential Test evidence demonstrated that no reasonably available lower-risk alternative sites had been identified for a comparable residential development within the relevant search area.

The Exception Test evidence then explained how the development could be made safe for its lifetime, with residual flood risk managed through design, refuge, warning and drainage measures.

This gave the planning submission a clearer flood risk policy position. Rather than relying only on technical mitigation, the application was supported by a structured explanation of why the site was sequentially appropriate and how the remaining risk would be managed.

For the applicant, this reduced the risk of avoidable delay and gave the planning authority a more complete basis for assessing the proposal. This project shows why Sequential Test evidence needs to be treated as a planning argument, not a map-based formality.

FAQ:

Why was a Sequential Test needed for this London residential site?

The site was in Flood Zone 3 and the proposal was for residential use, so the planning submission needed to show whether any reasonably available lower-risk sites could accommodate a comparable development.

Can residential development be acceptable in Flood Zone 3?

Yes, but only where planning policy requirements are properly addressed. The Sequential Test must be considered first, and the Exception Test may also be needed depending on the vulnerability of the proposed use.

Why did the search area matter?

The search area had to reflect the type and scale of development proposed. For a small London residential scheme, the assessment needed to consider realistic comparable sites, not unrelated strategic development opportunities.

What made the Sequential Test evidence stronger?

The assessment separated three issues: whether lower-risk alternatives were reasonably available, whether the site could be made safe, and how residual flood risk would be managed through design and drainage.

How did the Exception Test support the planning case?

The Exception Test explained why the development could be made safe for its lifetime without increasing flood risk elsewhere, using measures such as safe refuge, resilient construction and surface water management.