Which reports a scheme needing both flood tests needs

A scheme that engages both the sequential test and exception test reports is one where the flood zone and the vulnerability of the proposed use combine to make the development inappropriate without further justification. The commonest case is housing, a more vulnerable use, in Flood Zone 3a. The second is a highly vulnerable use such as basement accommodation in Flood Zone 2. In both, three documents have to work together: a sequential test, an exception test and a site-specific flood risk assessment, and they answer different questions for different readers.

When both tests are engaged

The compatibility tables in the Planning Practice Guidance on flood risk and coastal change set the position. They cross-reference five vulnerability classifications against the flood zones, and the cells tell you whether a use is appropriate, needs the exception test, or is not permitted.

Some proposals are outside the sequential test altogether. Development on a site allocated in an adopted local plan where the test was applied at plan-making stage, householder extensions, and most changes of use are exempt, though a change of use to a caravan or camping site, or one involving a basement, is not. Check the exemptions before you commission anything, because applying a test you did not need to apply invites the council to scrutinise an answer you did not have to give.

The flood zone comes from the Environment Agency Flood Map for Planning, and the council's Strategic Flood Risk Assessment will often refine it with modelling the national map does not reflect. Use both, and say which you relied on.

What the sequential test must demonstrate

The sequential test is a planning judgement, not a hydraulic calculation, and it is the document most often written badly. It has to show that there are no reasonably available sites appropriate for the proposed development in areas at a lower risk of flooding.

Four things have to be settled before the writing starts. The area of search, which the local planning authority defines and is normally the authority's area or a defined functional area rather than a radius the applicant picks. The development to be accommodated, described by use and quantum rather than by the applicant's business plan. What "reasonably available" means, which covers availability, suitability and achievability. And the date of the search.

Then the evidence. Sites from the authority's land availability assessment and brownfield register, allocated sites not yet delivered, sites marketed in the search period, and a reasoned sieve showing why each is not reasonably available. A Sequential Test (Flood) that lists three nearby sites and dismisses them in a sentence each will fail, and the Environment Agency will say so in its consultation response.

The honest difficulty is that a well-run sequential test can conclude against the applicant. That is the point of it, and a test written to a predetermined answer reads that way to an experienced officer.

What the exception test adds

The exception test has two limbs and both must be satisfied. First, the development must provide wider sustainability benefits to the community that outweigh the flood risk. Second, it must be safe for its lifetime, taking account of the vulnerability of its users, without increasing flood risk elsewhere and, where possible, reducing flood risk overall.

The first limb is where applicants over-claim. Benefits that count are specific and attributable: affordable housing in quantified terms, remediation of contaminated land, delivery of a community facility, regeneration of a defined brownfield site, provision of flood defence benefiting third parties. Benefits that do not count include general contribution to housing supply and the applicant's own commercial return.

The second limb is engineering, and it is answered by the flood risk assessment rather than by argument. An Exception Test (Flood) document pulls the two together, cross-referring to the technical work for safety and setting out the benefits case with evidence for each item.

Both limbs have to pass. A scheme with unanswerable sustainability benefits that cannot be made safe fails, and so does a demonstrably safe scheme with nothing to offer the community.

Vulnerability classification and why it decides this

Vulnerability is the input that determines everything else, and it is sometimes mis-assigned.

Highly vulnerable covers basement dwellings, caravan and mobile home sites intended for permanent residential use, and installations requiring hazardous substances consent. More vulnerable covers hospitals, residential institutions, dwellings, hotels, student halls, drinking establishments and non-residential uses for health, education and community facilities. Less vulnerable covers most shops, offices, general industry, storage and agriculture. Water-compatible covers flood defence infrastructure, water transmission, docks and amenity open space. Essential infrastructure covers transport infrastructure in a functional floodplain and strategic utility installations.

Two points catch people. A dwelling with habitable accommodation in a basement is highly vulnerable even where the rest of the house is above the flood level, which is why Flood Zone 3a rules out basement bedrooms. And a mixed-use scheme takes the classification of its most vulnerable element unless the elements can be separated by level, which is the standard design response: commercial at ground, residential above.

The site-specific assessment that supports both

A Flood Risk Assessment is the technical spine, and for a scheme carrying both tests it has to cover more than the zone.

Assess all sources: fluvial, tidal, surface water, groundwater, sewer surcharge and reservoir failure where relevant. Establish site levels from a measured topographic survey rather than from national terrain data. Give design flood levels, depths, velocities and the resulting hazard rating for the 1 per cent annual probability event and the appropriate climate change allowance, which the Environment Agency publishes by river basin district and epoch and revises, so cite the version used.

Then the safety case. Finished floor levels with a stated freeboard above the design level, flood resistance and resilience measures, safe access and egress assessed against the depth and velocity on the escape route, flood warning arrangements and an evacuation plan, residual risk including breach and overtopping where the site is defended, and compensatory storage on a level for level basis where development is proposed in the floodplain.

Where the site sits behind defences, breach modelling is usually unavoidable. Officers and the Environment Agency will not accept a defended-case assessment alone.

Drainage evidence running alongside

Flood risk to the site and flood risk from the site are separate questions, and the second one is the drainage strategy.

A Drainage Strategy should set the discharge hierarchy, infiltration first where ground conditions and groundwater levels allow, then a watercourse, then a sewer with the undertaker's capacity confirmation. Restrict discharge to the greenfield rate for the relevant return period, size attenuation for the critical duration with an allowance for climate change and urban creep, and show exceedance routes for events beyond the design standard.

The lead local flood authority is a statutory consultee on major development under the arrangements flowing from the Flood and Water Management Act 2010, and the Schedule 3 approval regime for sustainable drainage is not yet in force, so approval still runs through the planning condition rather than a separate consent. Verify that position before writing it into a strategy. Where a public sewer crosses the site, the undertaker's requirements add constraints, as covered in documents for a site crossed by a public sewer.

Assembling the three documents as one case

Submit them as three separate documents, not as chapters of one report, because three different readers use them. The planning officer reads the sequential test. The Environment Agency reads the flood risk assessment. The committee reads the exception test benefits case.

Keep the numbers identical across all three. Different design flood levels in the assessment and the exception test is the single most common reason for a holding objection, and it is entirely avoidable.

Sequence the work so the flood risk assessment is drafted first, since the safety conclusion feeds the exception test and the vulnerability finding may change the quantum you take through the sequential test. Then write the sequential test, then the exception test last.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. You can scope the full bundle on the planning documents index.