Climate Change Allowances for Flood Risk Assessments: Which One Do You Use?

Climate change allowances are the uplift figures the Environment Agency expects you to build into a flood risk assessment so a development stays safe for its whole lifetime rather than only on the day it opens. There are four of them. Which one you use, and which percentile and which epoch you read it from, depends on where the site is, what is being built and how long it will stand.

This guide sets out each allowance on its own terms. They do not share a structure. Peak river flow offers three percentiles across three epochs; peak rainfall offers two percentiles across two different epochs; sea level offers two percentiles across four epochs again. Getting those crossed is the commonest technical error we see in submitted assessments, and the one the Environment Agency picks up fastest. It is also entirely avoidable.

Every allowance ends up in one of two documents: the flood risk assessment that shows a development is safe for its lifetime, or the drainage strategy that shows its runoff is controlled. Which allowance you need follows from which of those you are producing. On most sites the answer is both.

The figures have not moved since 2022. The Environment Agency guidance was last updated on 27 May 2022, so nothing published since (the August 2026 NPPF included) changes the percentages you read from the maps. What changed is how much now turns on reading them correctly.

What are climate change allowances?

Environment Agency climate change allowances are predictions of anticipated change that you add to present-day flood risk so a development remains safe across its design life. The guidance lists four:

• Peak river flow. The anticipated increase in the peak flow a river carries, which drives how high and how far it floods. Published by management catchment.
• Peak rainfall intensity. The anticipated increase in short, heavy rainfall, which drives surface water flooding and the size of your drainage. Also by management catchment.
• Sea level rise. The anticipated rise in mean sea level, published for six areas of England and four epochs, for coastal and tidal sites.
• Offshore wind speed and extreme wave height. A single combined allowance for coastal wave modelling, applied where waves and surge matter.

The first two do most of the work inland. Peak river flow sets the modelled flood levels in a flood risk assessment for planning; peak rainfall sets the storage volumes in a surface water drainage strategy. Sea level and wave height come in on the coast. On a tidal site they often matter more than either of the inland pair, because a rise in still-water level lifts every extreme event on top of it rather than changing how often the extremes arrive.

Each allowance is offered as a range rather than a single number, because the future climate is a range of possibilities. The range is expressed in percentiles, and the percentile you are expected to design to is set by the flood zone, the vulnerability of the development and, for rainfall, its lifetime. The underlying projections are the Met Office's UKCP18 set (UK Climate Projections), run on a high-emissions pathway. That is a deliberately cautious basis the Environment Agency uses for planning, not a forecast of what will happen.

When do you need to apply climate change allowances?

You need them whenever a site is at risk of flooding now, or could be over the lifetime of the development. Policy F4 of the August 2026 NPPF puts it in those terms: a flood risk assessment is required wherever a site is at risk from any source of flooding, now or in the future. That pulls in Flood Zone 1 land that climate change could move into Zone 2 or 3, and it makes the rainfall allowance relevant to drainage design almost everywhere, because rainfall is intensifying across all of England.

The Environment Agency checks the allowances when it advises as a statutory consultee, and the planning practice guidance on flood risk and coastal change sets the framework it works to. Leaving them out, or using a superseded figure, is one of the more common reasons an assessment comes back with an objection. If you are commissioning the work rather than doing it, our flood risk assessment for planning service applies the right allowance at the outset and defends it if the Environment Agency queries it.

Residential development should be treated as having a minimum lifetime of 100 years. That single assumption pushes almost every housing scheme into the longest epochs and the fuller allowances.

Lifetime is what decides which epoch you read from. Establish it early. A short-life commercial building and a permanent housing scheme on the same street can legitimately use different allowances, because they will not be standing for the same length of time. Our guide on when you need a flood risk assessment sets out the triggers, and how to do a flood risk assessment shows where the allowances sit in the wider report.

Which allowance, which percentile, which epoch?

There is no single answer. The reason trips up more assessments than any other point on the topic, and it is this: the four allowances use different percentile sets and different epoch boundaries. Peak river flow has three named percentiles. Sea level has no central allowance at all. Peak rainfall has no higher central. Reading the wrong row of the wrong table produces a number that looks plausible and is wrong. Nothing on the page warns you.

• Central allowance. The 50th percentile. Available for peak river flow and peak rainfall. There is no central sea level allowance.
• Higher central allowance. The 70th percentile. Available for peak river flow and sea level. There is no higher central rainfall allowance.
• Upper end allowance. The 95th percentile. Available for peak river flow, peak rainfall and sea level.

How the percentiles work

The percentile describes the proportion of the modelled projections that fall at or below the allowance. An allowance at the 50th percentile is exceeded by 50% of the projections in the range; at the 70th it is exceeded by 30%; at the 95th, by 5%. A higher percentile is a more precautionary figure, not a more likely one. That distinction matters when a planning officer asks why you designed to it, because the answer is about the consequence of being wrong over a hundred years rather than about what the model expects to happen. The glossary entry on central, higher central and upper end allowances covers the terminology.

Why the epochs do not line up

The epochs are where the real confusion sits. Each allowance runs on its own calendar, and the label "2050s" means two different spans depending on which dataset you have open. The peak river flow epochs are defined in the published peak river flow dataset rather than on the guidance page itself, which is part of why the clash goes unnoticed.

The 2050s epoch means 2040 to 2069 for peak river flow and 2022 to 2060 for peak rainfall. Two government datasets, one label, two different spans, and no warning on either.

Peak river flow allowances

Peak river flow allowances give the anticipated change to a river's peak flow as a percentage uplift, published by management catchment. You read the figure for your catchment, apply it in the river model, and the model returns climate-change-adjusted flood levels. Management catchments are sub-catchments of the river basin districts, so the figure is far more local than the single national number it replaced.

The current figures were rebuilt in July 2021 using UKCP18, on the basis set out in the Environment Agency's research on climate change and fluvial flood peaks, and are published in the peak river flow allowances by management catchment. Which percentile you assess depends on the flood zone and the flood risk vulnerability classification of what is being built.

The four rules that sit alongside the table

Four rules sit alongside that table and are routinely missed. Where the local planning authority considers a development acceptable even though it does not follow the flood zone compatibility categories, you use the higher central allowance. Strategic flood risk assessments, prepared to the Environment Agency's SFRA guidance, assess both the central and the higher central. Safe access, escape routes and places of refuge are designed to the central allowance for everything except essential infrastructure, which uses higher central. And where you are assessing off-site impacts or floodplain storage compensation, it is central for most cases and higher central where the affected area contains essential infrastructure.

The allowances also apply to sites currently in Flood Zone 1 where the strategic flood risk assessment shows they may fall in Zone 2 or 3 in future. A dry site today is not a reason to skip the question.

Applying the uplift is a modelling exercise, not a spreadsheet one, and the Environment Agency's technical standards govern how it is done. Where a scheme needs the flood levels re-derived rather than read off a map, our 1D and 2D hydraulic flood modelling team builds the climate change scenarios to the EA river modelling technical standards.

Peak rainfall intensity allowances

Peak rainfall intensity allowances give the anticipated increase in short, heavy rainfall, and they matter most for surface water risk and drainage design. They are published by management catchment for both the 1% and the 3.3% annual exceedance probability events, across two epochs, and they offer only two percentiles: central and upper end. The current set dates from 10 May 2022 and is derived from UKCP Local 2.2km projections and the FUTURE-DRAINAGE research, replacing the flat 40% national uplift that preceded it. The figures are published as peak rainfall allowances by management catchment.

Which rainfall figure applies to your development?

Which figure you use is set by lifetime, and the guidance gives three cases rather than two.

• Lifetime up to 2060. Central allowance, 2050s epoch (2022 to 2060), for both the 1% and 3.3% AEP events.
• Lifetime between 2061 and 2100. Central allowance, 2070s epoch (2061 to 2125), for both events.
• Lifetime beyond 2100. Upper end allowance, 2070s epoch, for both events. A housing scheme assessed on a 100-year lifetime lands here.

There is one exception worth knowing, and it belongs to rainfall alone. In some catchments the 2050s allowance is higher than the 2070s. Where that happens and the development has a lifetime beyond 2061, you use the higher of the two. No equivalent rule exists for peak river flow.

Choosing the climate change allowance for drainage design

Scope matters too. The rainfall allowances are for site-scale work such as drainage design, and for surface water mapping in catchments smaller than 5 square kilometres and in urbanised drainage catchments. For larger rural catchments, direct rainfall modelling is unlikely to be appropriate and the fluvial risk should be assessed with the peak river flow allowances instead. The two are not interchangeable: the guidance is explicit that peak river flow allowances must not be used to adjust rainfall totals.

This is the allowance that most often reshapes a scheme. A higher rainfall figure means more storage: larger tanks, bigger basins, or more permeable surface. It is usually cheaper to find that out at strategy stage than at discharge of conditions.

Under the 2025 National Standards for Sustainable Drainage Systems, which Policy F8 of the August 2026 NPPF now makes a planning requirement, new drainage designs are expected to use the upper end rainfall allowance, with an urban creep allowance applied on top of impermeable areas. If you are sizing attenuation storage against those standards, our surface water drainage strategy service builds the allowance into the design from the start, and our climate change and urban creep calculator gives a quick sense of the effect on runoff before you commit. Our explainer on how increasing rainfall is changing flood risk covers the attribution science behind the figures.

Sea level rise allowances

Sea level allowances give the anticipated rise in mean sea level in millimetres per year, published for six areas of England across four epochs, against a 1981 to 2000 baseline. They offer two percentiles only: higher central and upper end. There is no central sea level allowance. On both flood risk assessments and strategic flood risk assessments you are expected to assess both of the ones that exist, which is a meaningful difference from the fluvial case, where a single central figure carries most schemes and the higher central is reserved for essential infrastructure.

The totals differ around the coast because the land is still moving after the last ice age, a process called glacial isostatic adjustment: the north of the country is slowly rising and the south slowly sinking, so the same rise in the sea produces a different rise relative to the shore.

Between roughly one metre and just over one and a half metres of total rise to 2125, depending where you are on the coast and which of the two allowances applies.

Rising sea levels also accelerate coastal erosion, so a tidal or coastal assessment often has to consider shoreline change as well as still-water level. Our explainer on human-driven sea level rise and UK coastal flooding sets out the observed trend, and planning for sea level rise beyond 2100 looks at what happens when a scheme's horizon runs past where the allowances stop.

Offshore wind speed and extreme wave height allowances

On an exposed coast, waves and surge can matter as much as still-water level. The Environment Agency publishes a combined allowance for offshore wind speed and extreme wave height, to be used where those effects are not already built into an Environment Agency coastal model. It runs on two epochs of its own. It is the simplest of the four to read.

• 2000 to 2055. A 5% allowance for both offshore wind speed and extreme wave height.
• 2056 to 2125. A 10% allowance for both.
• Sensitivity test. 10% in both epochs, applied where a credible maximum scenario is being tested.
• Not cumulative. Use the epoch the development's lifetime falls into. The allowances do not add together across epochs.

Storm surge is treated separately and is generally already captured by the present-day extreme sea levels a coastal model starts from, so it needs a separate addition only in a credible maximum scenario. The present-day baseline itself comes from the Coastal Flood Boundary extreme sea levels dataset.

How to find your allowance on the Environment Agency maps

The quickest route to the right figure is the Environment Agency's climate change allowances map, of which there are in fact two: one for peak river flow and one for peak rainfall. Search the site, read off the management catchment, and the allowances appear in a pop-up. The same figures are published as tables if you already know your catchment.

1. Open the right map. Peak river flow for fluvial risk, peak rainfall for surface water and drainage design. They are separate tools with separate figures.
2. Locate the site by postcode, national grid reference or place name in the find-address box.
3. Select the management catchment and note every allowance shown, for each epoch and each percentile, rather than only the one you expect to need.
4. Check the dominant flood source. Where the main risk at the site comes from a neighbouring catchment, use that catchment's allowance instead.
5. Match the epoch to the development's lifetime, then choose the percentile from the rules above. Record which map, which epoch and which percentile you used, because the Environment Agency will ask. Its guidance on flood risk assessments for planning permission and its standing advice both set out what it expects to see.

Record the epoch alongside the percentage. An allowance quoted without its epoch and percentile cannot be checked, and an assessment the Environment Agency cannot check is an assessment it will query.

What the Flood Map for Planning now shows

The Flood Map for Planning now shows some of this without any modelling at all. Since 28 May 2026 it carries surface water flood risk with climate change included, built on the upper end rainfall allowance for the 2070s epoch, the same precautionary figure the 2025 SuDS standards use. Our summary of the May 2026 Flood Map for Planning update walks through the new layers, and the Flood Map for Planning explainer covers what it does and does not show.

A worked example: housing in Flood Zone 3a

Take a proposed housing scheme on a river, in Flood Zone 3a, with a 100-year lifetime. Housing is "more vulnerable" under Annex F of the NPPF, so the sequence runs as follows.

Look at what that means. The same scheme reads from two different epochs at once: the 2080s epoch for the river model and the 2070s epoch for the drainage design. That is correct, and it is exactly the point at which a reviewer who has only ever used one of the two datasets starts asking questions.

With the uplift in hand, the modelling gives a climate-change-adjusted design flood level. Finished floor levels are then set above it with freeboard, and the scheme has to show it does not worsen risk elsewhere. Swap the housing for a substation and the percentile steps up to higher central; move the site to the coast and you are assessing higher central and upper end sea level instead. The method holds. Only the inputs change.

Where the outcome is finely balanced, run the next percentile up as a sensitivity test. Showing that the scheme still works under a higher allowance is often what settles an Environment Agency query, and it is cheaper than an Environment Agency objection after submission.

Credible maximum scenarios and the H++ allowance

Some proposals have to be tested against a deliberately extreme future. Nationally significant infrastructure projects, new settlements and significant urban extensions may need a credible maximum, or high-impact, climate change scenario as a sensitivity test alongside the standard assessment.

The H++ sea level figure is a total rise of 1.9 metres to 2100. There is no H++ value beyond 2100, which is a real constraint on any scheme whose lifetime runs past the end of the century, and the reason a growing number of coastal appraisals now look to the research rather than the guidance for that later horizon.

Running these scenarios does not usually mean designing to the worst case today. It tests how sensitive a scheme is to a changing climate, which is what supports an adaptive approach: leaving room to raise defences, add storage or adjust the layout later if the climate moves faster than the central projection. Setting a building back from a river so its defences can be improved in future is the simplest version of it.

What did the August 2026 NPPF change?

Nothing in the allowance figures changed. The Environment Agency's guidance sits outside the National Planning Policy Framework and was last revised in 2022, so the percentages you read from the maps are untouched by the Framework published on 17 August 2026. What changed is the weight the planning system now puts on getting them right. Four of the new Chapter 18 policies decide where the allowances apply and what happens when they are wrong.

• Policy F4. A flood risk assessment is required wherever a site is at risk from any source of flooding, now or in the future, so more Flood Zone 1 sites need a climate-aware assessment than before.
• Policy F5. Sites at risk of surface water flooding only can avoid the sequential test where the assessment shows the development is safe for its lifetime. The allowances inside that assessment carry the decision.
• Policy F7. Development that cannot be shown to be safe for its lifetime should be refused. A superseded or missing allowance now invites refusal rather than an objection.
• Policy F8. Sustainable drainage to the 2025 National Standards applies to all development, which brings the upper end rainfall allowance into everyday drainage design.

The shift is in the wording, and it is the whole story. Chapter 18 moved from "should only be permitted" to "should be refused unless", and an allowance is now part of what the applicant has to prove rather than part of what the consultee might comment on.

What changed for essential infrastructure

One change bears directly on which percentile you read. The vulnerability classification now sits in Annex F of the NPPF rather than the old Annex 3, and the essential infrastructure category has grown: data centres, electric vehicle charging infrastructure and hydrogen infrastructure are now classed as essential, so they assess the higher central allowance rather than the central one. The full classification is in our guide to flood risk vulnerability classification, and the zones themselves in flood zones explained.

Our full analysis of the August 2026 NPPF flood risk and drainage changes covers Chapter 18 in detail, the surface water sequential test exemption explains Policy F5, and our NPPF flood risk guidance service page covers what it all means for a live application. For the tests themselves, see our sequential and exception test reports.

Do the allowances apply in Wales and Scotland?

No. The Environment Agency's climate change allowances apply in England only, and both Wales and Scotland publish their own figures on their own structures. Using an English allowance across the border is a straightforward error. So is assuming the terminology carries over. Wales calls its 95th percentile the "upper" allowance, not the "upper end". Welsh sites are also mapped on Natural Resources Wales' Flood Map for Planning, not the English one.

Wales climate change allowances under TAN15

In Wales, flood risk is assessed through a flood consequences assessment under TAN15, which was published on 31 March 2025 and is the current instrument. The Welsh Government then issued new climate change allowances in Flood Consequences Assessments: climate change allowances in March 2026, replacing the September 2021 guidance and applying to planning applications submitted from 1 April 2026. They draw on the same UKCP18 projections as the English figures but are structured by river basin district and use a different percentile vocabulary. Our guides to the new TAN15 and TAN15 explained set out the Welsh system, and we prepare flood consequence assessments for planning in Wales.

Wales changed its allowances in March 2026 and England did not. An assessment that quietly applies English figures to a Welsh site is now wrong twice over: wrong nation, and wrong year.

SEPA climate change allowances in Scotland

In Scotland, SEPA publishes its own climate change allowances on a structure that has no English equivalent: a single total percentage change to a target year for each River Basin Region, with no epochs and no menu of percentiles. The current document is version 6, issued in February 2025. Scottish applications are assessed against SEPA's flood maps rather than the Environment Agency's, and the figures themselves differ substantially from the English ones.

Development allowances are not scheme allowances

One further distinction catches people out within England. The allowances covered here are the ones for development. The Environment Agency publishes a separate set for flood and coastal risk projects, schemes and strategies, with different figures and different application rules. They are not interchangeable. Quoting one in place of the other is a quick way to lose a reviewer's confidence, and it happens often enough that an experienced Environment Agency officer will check which set a figure came from before reading anything else in the chapter.

Frequently asked questions

Getting the allowance right is a small part of a flood risk assessment that carries a disproportionate amount of the risk: it is quick to check, quick to challenge, and one of the first things a reviewer looks at. If you would rather not carry that risk, our flood risk assessments for planning page sets out how we scope the work and what to expect, and our guide to whether you need a flood risk assessment and a drainage strategy covers where the two overlap.