Choosing the right climate change allowance epoch
Most climate change allowance errors are selection errors rather than modelling errors. Getting the epoch, the catchment and the category right first time avoids a rerun.
Two choices decide whether an Environment Agency consultation response accepts your flood modelling: the epoch you assess against, and the allowance category you apply within it. Both are selections rather than calculations, and both are set out in the Agency's published climate change allowances guidance, which is revised periodically and keyed to river basin districts and management catchments.
Get either wrong and the model has to be rerun, which on a main river site with a detailed hydraulic model is weeks rather than days.
Where the allowances live and how often they change
The allowances are published on gov.uk as guidance for flood risk assessments, covering peak river flow, peak rainfall intensity, sea level rise, offshore wind speed and extreme wave height. They were restructured in 2022 so that peak river flow and peak rainfall figures are given by management catchment rather than by region, which was a change of granularity as well as of numbers.
Treat the guidance as a live document. Record in the assessment the date of the version you used, the river basin district, the management catchment name and the allowance category, and keep a dated copy of the page. When a reviewer asks two years later which figures you applied, that record is the answer.
Choosing the epoch from the development lifetime
The epoch follows the lifetime of the development, not the length of the lease or the client's investment horizon.
The usual conventions are a 100 year lifetime for residential development and a 60 year lifetime for most commercial and industrial development, with essential infrastructure often assessed over a longer period again. Apply the lifetime from the expected date of completion, then select the epoch that covers the end of that period.
| Development | Assumed lifetime | Epoch normally applied |
|---|---|---|
| Residential | 100 years | The later epoch in the guidance |
| Commercial and industrial | 60 years | Depends on completion date, often the later epoch |
| Temporary use with a fixed end date | As permitted | The epoch covering that date |
| Essential infrastructure | Asset design life, often longer | The later epoch, sometimes with sensitivity testing |
Where a scheme spans both, assess to the longer lifetime. A mixed use block with flats above commercial floorspace is residential for this purpose.
State the assumption explicitly in the assessment. Reviewers query unstated lifetimes more often than they query the arithmetic, and a sentence naming the completion year and the resulting end date closes the point.
Central, higher central and upper end
Within each epoch the guidance gives a range of allowances describing how much uncertainty you are choosing to absorb. The categories are central, higher central and upper end, and they are not interchangeable.
Selection is driven by the flood zone and the flood risk vulnerability classification of the development, using the classification table in the flood risk planning practice guidance: essential infrastructure, highly vulnerable, more vulnerable, less vulnerable and water compatible. The Agency's guidance sets out which category to apply to each combination, and it also asks for more than one category to be assessed in some cases so that the sensitivity of the scheme is visible.
Take the combination from the current table rather than from a previous project. The mapping has changed between versions, and an allowance that was correct on a scheme submitted three years ago may not be correct now.
One practical point on completion dates. Where a scheme is phased over several years, take the completion date of the last phase rather than the first, because the epoch has to cover the whole development. Outline permissions with long reserved matters periods need the same treatment, and it is worth saying in the assessment which date you used and why.
Peak river flow against peak rainfall intensity
These answer different questions and both are usually needed.
Peak river flow allowances uplift the modelled flow in a watercourse and drive fluvial flood extents, levels and finished floor levels. They belong in the hydraulic modelling that supports the Flood Risk Assessment.
Peak rainfall intensity allowances uplift the design storm used in the surface water drainage calculation, and they drive attenuation volumes, pipe sizing and exceedance routing. They belong in the Drainage Strategy and are checked by the lead local flood authority rather than the Agency, which is one of the items covered in what the lead local flood authority checks in a drainage strategy.
A common error is applying a peak river flow allowance to a rainfall runoff calculation, or using a single uplift for both. They are derived differently and the figures differ, sometimes substantially.
Tidal sites need the sea level allowance applied cumulatively year by year to the assessment date, and coastal sites may also need the offshore wind and wave allowances. Where a site is influenced by both a river and the tide, the joint probability case has to be agreed with the Agency before modelling starts.
Sea level allowances are cumulative rather than a single uplift, so they are applied year by year from a baseline to the assessment date. That arithmetic is easy to get wrong in a spreadsheet, and it is one of the first things a coastal reviewer checks.
What to record in the assessment
A climate change section that survives review states the lifetime assumption and its basis, the completion year, the epoch selected, the river basin district and management catchment, the vulnerability classification and flood zone, the allowance category applied and why, the numerical allowance used, and the version date of the guidance.
Set it out as a short table near the front of the flood risk chapter rather than burying it in an appendix. The Agency's reviewer looks for it first, and finding it immediately shortens the response. The structure of that section is dealt with in more detail in the climate change allowances section of an FRA.
Where the development includes below ground accommodation, the residual risk case needs to reflect the same allowances, and the specific tests are covered in basement development and flood risk.
When the allowances are revised mid-application
This happens, and there is no automatic rule. The starting position is that an assessment prepared under the guidance current at the date of submission is not invalidated by a later revision, but the Agency can and does ask for a sensitivity check where the change is material and the application is still undetermined.
Three practical steps reduce the pain. Build the model so that the uplift is a parameter rather than hard coded, so a rerun is quick. Include a short sensitivity paragraph testing the next allowance category up, which often demonstrates that the scheme is not sensitive to the change. And where an application is likely to run long, ask the Agency in the pre-application response to confirm what it expects if the figures move.
On appeal the position is firmer. An inspector decides on the material considerations at the date of the decision, so an appeal running eighteen months after submission will be judged against the guidance in force then.
This is general guidance, not legal advice. Check the position with the Environment Agency, your lead local flood 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.
Frequently asked questions
Which epoch should I use for a housing scheme?
Apply a 100 year lifetime from the expected completion date and select the epoch that covers the end of that period, which for schemes completing now means the later epoch in the published guidance. State the completion year and the resulting end date in the assessment so the reviewer can see the basis for the choice.
What is the difference between central and upper end allowances?
They describe how much of the projected range you are designing against, with upper end the most precautionary. Which one applies is driven by the flood zone and the flood risk vulnerability classification of the development, using the combination set out in the current Environment Agency guidance rather than professional preference.
Can I use the same allowance for river flow and rainfall?
No. Peak river flow allowances uplift flows in a watercourse and drive fluvial levels. Peak rainfall intensity allowances uplift the design storm and drive attenuation volumes and pipe sizing. They are derived differently, the figures differ, and applying one to the other is a common reason for a consultation objection.
What happens if the allowances change after I submit?
An assessment prepared under the guidance current at submission is not automatically invalidated, but the Environment Agency may ask for a sensitivity check while the application is undetermined. On appeal, the inspector decides on the position at the date of the decision, so a long running case will be judged against the newer figures.
Where do I find the allowance for my catchment?
The Environment Agency publishes peak river flow and peak rainfall intensity allowances by river basin district and management catchment in its climate change allowances guidance on gov.uk. Identify the management catchment for the site first, then read the epoch and category from the relevant table.
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.