Climate change allowances: picking the right percentile
Four inputs decide which climate change allowance applies: catchment, variable, vulnerability and lifetime. Getting one wrong costs you the floor levels or the storage volume.
Picking a climate change allowance is a decision with four inputs: which management catchment the site drains to, which allowance set applies to the variable you are modelling, how vulnerable the development is, and how long it will stand. Get any one of them wrong and the finished floor levels, the attenuation volume or both will be wrong, and the Environment Agency will say so in its consultation response.
The allowances are published by the Environment Agency and they are revised. Working from a figure a colleague used on a scheme two years ago is the commonest way to earn a holding objection.
Where the allowances live and how often they change
The allowances sit in the Environment Agency guidance on flood risk assessments and climate change allowances, published on gov.uk. They cover peak river flow, peak rainfall intensity, sea level rise, offshore wind speed and extreme wave height.
They have been revised more than once since they were first issued, and the structure has changed as well as the numbers. Peak rainfall allowances in particular moved from a national figure to catchment-based values, which means a design carried over from an older scheme in a different catchment may be using a figure that was never right for this site.
Record three things in the flood risk assessment every time: which allowance set you used, the date you took it from the guidance, and the management catchment it relates to. That single paragraph resolves most reviewer queries before they are asked.
Peak river flow allowances by catchment
Peak river flow allowances are published by management catchment and expressed as a percentage uplift on the present day peak flow, banded by time period and by allowance category. The categories run from central through higher central to upper end, and the spread between them can be wide in some catchments and narrow in others.
Which band you use is not a free choice. It follows from the flood zone and the vulnerability of the development, and the guidance sets out the combinations. Essential infrastructure and highly vulnerable development in a high risk zone attract the more onerous allowances. Less vulnerable development in a lower risk zone does not.
Where the site is defended, the allowance also affects the residual risk case. A defence overtopping scenario modelled on a central allowance and a breach scenario modelled on an upper end allowance tell different stories about safe access and refuge levels, and the reviewer will want both.
Peak rainfall allowances for surface water design
Surface water design uses the peak rainfall allowances, and these are the ones most often applied incorrectly on small sites.
The allowances are published for different annual exceedance probabilities and different time periods, again by management catchment, with central and upper end values. A Drainage Strategy should state the allowance applied to each design event and show that the attenuation volume was derived from rainfall uplifted accordingly.
Two errors recur. Applying a river flow allowance to a rainfall calculation, which produces a number that looks plausible and is not the right one. And applying a single allowance to every return period modelled when the guidance distinguishes between them.
Where the receiving system is a public sewer, the water undertaker may have its own design requirement on top of the Environment Agency allowance. Ask rather than assume, because the two are not always aligned.
Choosing by vulnerability and lifetime
Development lifetime drives which time period you take the allowance from, and it is a judgement you have to state and justify.
| Development type | Lifetime usually assumed |
|---|---|
| Residential | 100 years from the date of construction |
| Commercial and industrial | 60 years unless a longer life is justified |
| Essential infrastructure | The design life of the asset, often well beyond 100 years |
| Temporary or time limited use | The period of the permission, stated in the assessment |
The vulnerability classification comes from the flood risk planning practice guidance, which sets out the categories from water compatible through less vulnerable and more vulnerable to highly vulnerable and essential infrastructure. It decides both which allowance band applies and whether the development is capable of being permitted in the flood zone at all.
State the lifetime and the classification in the opening pages of the assessment, with a line explaining the basis. A reviewer who has to hunt for them assumes the worst.
Sensitivity testing and when it is expected
Sensitivity testing means running the model with a more onerous allowance to see what breaks. It is expected on more vulnerable and essential infrastructure development, on sites where the consequences of being wrong are severe, and wherever the difference between allowance bands in that catchment is large.
What the test should report is not a second set of numbers but a conclusion about robustness. Does the finished floor level still provide freeboard above the modelled level? Does safe access remain dry, or dry enough, at the higher allowance? Does the attenuation still perform, and what is the half drain time under the more onerous rainfall? Our note on what a consultee checks in a drainage model output covers how that evidence is read.
Where the test shows the scheme fails at the upper end, say so and explain the mitigation rather than quietly reporting only the central case. Reviewers find the omission, and finding it costs you credibility on everything else in the document.
Recording the version used
Write the audit trail into the report, not into the modeller's file note.
State the guidance version and the date accessed, the management catchment name, the allowance category and the percentage applied for each variable, the time period selected, and the assumed development lifetime with its justification. Put it in a table near the front.
This matters beyond tidiness. Applications sit for months, allowances get revised mid-determination, and a report that clearly states what it used on a given date can be updated with a short addendum instead of being redone. A report that does not state it has to be rebuilt from scratch, usually at the point when the committee date is already fixed.
Consequences of using an outdated figure
An out-of-date allowance shows up as a number that does not match the reviewer's own check, and the response is a holding objection rather than a refusal. That sounds mild and is not, because it stops the determination clock in practice and it arrives at the end of the consultation period rather than the start.
The downstream effects are worse where the design has already been fixed. Finished floor levels set from a superseded river flow allowance may need raising, which changes level access, ramp lengths, storey heights and sometimes the whole ground floor layout. An attenuation volume derived from an old rainfall figure may not fit in the space reserved for it.
A Flood Risk Assessment should therefore be written so the allowance inputs sit in one identifiable place and can be changed without rewriting the narrative. Where the sequential approach to site selection is in play, our note on flood risk and betterment in the planning balance covers how the evidence is weighed.
Browse all 66 report types to see the full flood and drainage bundle a site may need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
This is general guidance, not legal advice. Climate change allowances are revised periodically, so check the current published schedule on gov.uk at the time you carry out the work.
Frequently asked questions
Where do I find the current climate change allowances?
In the Environment Agency guidance on flood risk assessments and climate change allowances, published on gov.uk. It covers peak river flow, peak rainfall intensity, sea level rise, offshore wind speed and extreme wave height. The figures and the structure have both been revised more than once, so take them fresh rather than from an earlier scheme.
Which allowance band should I use?
It follows from the flood zone and the vulnerability classification of the development rather than being a free choice. Essential infrastructure and highly vulnerable development in higher risk zones attract the more onerous bands. The guidance sets out the combinations, and the vulnerability categories come from the flood risk planning practice guidance.
What development lifetime should I assume?
Residential development is usually assessed over 100 years from construction and commercial or industrial development over 60 years unless a longer life is justified. Essential infrastructure follows the design life of the asset. For a time limited permission, use the period of the permission and say so. State the assumption and its basis near the front of the assessment.
When is sensitivity testing expected?
On more vulnerable and essential infrastructure development, on sites where the consequences of being wrong are severe, and wherever the gap between allowance bands in that catchment is wide. Report a conclusion about robustness rather than a second set of tables: whether freeboard, safe access and the attenuation performance all hold at the more onerous allowance.
What happens if I use a superseded allowance?
The reviewer's own check will not match and a holding objection follows, usually at the end of the consultation period rather than the start. If the design is already fixed, raising finished floor levels changes level access, ramps and storey heights, and an attenuation volume derived from an old rainfall figure may no longer fit its reserved space.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.