Climate Change Allowances for Flood Risk Assessments: A 2026 Guide

Climate change allowances are the figures the Environment Agency expects you to build into a flood risk assessment so that a development stays safe over its whole lifetime, not just on the day it is built. They convert what we know about a warming climate into numbers you can design to: more rain, falling harder; rivers carrying higher peak flows; and the sea rising along the coast. If your site is at risk of flooding now, or could be over the lifetime of what you are building, national planning policy expects those allowances to be applied, and the Environment Agency will look for them when it reviews your assessment.

This guide explains the current allowances in plain terms: what they cover, how they are structured around management catchments and percentiles, how to find the right figure for your site on the Environment Agency's maps, and which allowance to use for your development. It reflects the guidance in force in 2026 and replaces the older national percentages that many people still remember.

The right allowance depends on your site's management catchment, its flood zone and the vulnerability of what is being built. It is given as a range based on statistical percentiles, not a single national number.

What are climate change allowances?

Climate change allowances are predictions of anticipated change, published by the Environment Agency, that you add to present-day flood risk so a development remains safe across its design life. There are four of them, each aimed at a different flood mechanism.

• Peak river flow: the anticipated increase in the peak flow a river carries, driving how high and how far it floods. Given by management catchment.
• Peak rainfall intensity: the anticipated increase in short-duration rainfall, which drives surface water flooding and the size of your drainage. Also given by management catchment.
• Sea level rise: the anticipated rise in mean sea level, given by river basin district and epoch for coastal and tidal sites.
• Offshore wind speed and extreme wave height: allowances for coastal wave modelling, applied where waves and storm surge matter.

Each allowance is offered as a range across different future scenarios and different epochs, or periods of time, running out across the coming century. The reason for applying them is simple: a building or a piece of infrastructure has to cope not with today's climate but with the climate it will still be standing in decades from now.

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. That includes land currently shown in Flood Zone 1 where a strategic flood risk assessment suggests it might fall into Flood Zone 2 or 3 in future, and it includes surface water and drainage design almost everywhere, because rainfall is getting more intense across all of England.

The Environment Agency, as a statutory consultee, checks that you have used the allowances when it advises on a flood risk assessment or a strategic flood risk assessment. Getting them wrong, or leaving them out, is one of the more common reasons an assessment comes back with an objection.

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

Development lifetime is what decides which epoch you read from, so it is worth establishing 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 in more detail.

The four allowances explained

The four allowances answer four different questions about a site: how much more a river might flow, how much harder it might rain, how far the sea might rise, and how much bigger the waves might get. Peak river flow and peak rainfall do most of the work on inland sites; sea level and wave height come in on the coast.

Peak river flow allowances

Peak river flow allowances show the anticipated change to a river's peak flow, expressed as a percentage uplift by management catchment. Management catchments are sub-catchments of the larger river basin districts, so the figure is far more local than the old national number. You read the allowance for your catchment, then apply it in your river modelling to produce climate-change-adjusted flood levels.

The allowances were rebuilt using the UK Climate Projections (UKCP18) and are provided as a range of percentiles rather than one figure. Which end of that range you use depends on your flood zone and the vulnerability of the development, covered below.

Peak rainfall intensity allowances

Peak rainfall intensity allowances show the anticipated increase in short, heavy rainfall, and they matter most for surface water flood risk and for drainage design. They are given by management catchment for both the 1% and the 3.3% annual exceedance probability (AEP) events, and for two epochs. Use them for site-scale drainage design and for surface water mapping in small catchments (under 5 square kilometres) and urban drainage catchments.

This is the allowance that most often reshapes a scheme's surface water drainage strategy, because a higher rainfall figure means more storage: larger attenuation tanks, bigger basins or more permeable area. The 2022 update replaced the old flat 40% national uplift with allowances that vary from catchment to catchment.

Sea level allowances

Sea level allowances give the anticipated rise in mean sea level for each river basin district and epoch, in millimetres per year, with a cumulative total to 2125. They range from around a metre to over one and a half metres of total rise depending on the area, because the north of the country is slowly rising and the south is slowly sinking. On the coast you assess both the higher central and the upper end allowance.

Rising sea levels also increase the rate of coastal erosion, so tidal and coastal assessments often need to look at shoreline change as well as flood level. Our explainer on human-driven sea level rise and UK coastal flooding puts the trend in context.

Offshore wind speed and extreme wave height

On exposed coastal sites, waves and storm surge can matter as much as still-water level. The Environment Agency provides allowances for offshore wind speed and extreme wave height (5% to the 2050s and 10% beyond, with sensitivity tests of 10%), to be used where they are not already built into an Environment Agency coastal model. Storm surge is generally captured by the present-day extreme sea levels the models start from.

How the percentiles work: central, higher central and upper end

The allowances are given as a range because the future climate is a range of possibilities, not a single certainty. A percentile describes the proportion of modelled scenarios that fall below a given allowance, so a higher percentile is a more precautionary, less-likely-to-be-exceeded figure.

• Central allowance: the 50th percentile. Half the projections fall below it and half above, so it is exceeded by 50% of the modelled scenarios.
• Higher central allowance: the 70th percentile, exceeded by only 30% of the projections.
• Upper end allowance: the 95th percentile, exceeded by just 5% of the projections. This is the precautionary, credible-maximum figure.

The allowances draw on the high-emissions scenario within UKCP18, which is the deliberately cautious basis the Environment Agency uses for planning. The practical question on any site is not what the percentiles mean in theory, but which one you are expected to design to, and that is set by the flood zone and vulnerability rules.

How to find your climate change allowance

The quickest way to get the right figure is the Environment Agency's interactive allowance maps: one for peak river flow and one for peak rainfall. Search your site, read the management catchment, and the allowances appear in a pop-up. The same figures are published in the Environment Agency's allowance tables by management catchment if you already know your catchment.

1. Open the right map. Use the peak river flow map for fluvial risk and the peak rainfall map for surface water and drainage.
2. Locate your site. Search by postcode, national grid reference or place name in the find-address box.
3. Read the management catchment. Select it and note the allowances shown for each epoch and percentile in the pop-up.
4. Check the dominant flood source. Where the main risk comes from a neighbouring catchment, use that catchment's allowance instead.
5. Record epoch and percentile. Match the epoch to your development's lifetime, then pick the percentile from the rules below.

One point catches people out: in some catchments the allowance for the 2050s epoch is higher than for the 2070s. Where that happens and the development has a lifetime beyond 2061, you use the higher of the two.

Which allowance should you use?

For peak river flow, the percentile you assess is set by your flood zone and the flood risk vulnerability of the development. In broad terms, most development uses the central allowance, essential infrastructure uses the higher central allowance, and the upper end is reserved for credible-maximum scenarios. The table below summarises the benchmark the Environment Agency applies in flood zones 2 and 3a.

A few rules sit alongside the table. Strategic flood risk assessments assess both the central and higher central allowances. Safe access, escape routes and places of refuge are designed to the central allowance for everything except essential infrastructure, which uses higher central. For sea level you assess both higher central and upper end; for peak rainfall you use central for a lifetime up to 2100 and upper end beyond it. The classification that drives all of this is set out in our guide to flood risk vulnerability classification under NPPF Annex 3, and the zones themselves in flood zones explained.

A worked example

Take a proposed housing scheme in Flood Zone 3a, on a river, with a 100-year lifetime. Housing is "more vulnerable", so the sequence runs like this.

1. Confirm the flood source and zone. Fluvial risk, Flood Zone 3a, from the Environment Agency's Flood Map for Planning.
2. Classify the development. Dwellings are more vulnerable under NPPF Annex 3.
3. Find the allowance. Read the peak river flow allowances for the site's management catchment from the map.
4. Pick the epoch. A 100-year residential lifetime reaches the 2070s epoch (2061 to 2125).
5. Pick the percentile. More vulnerable development in Flood Zone 3a assesses the central allowance for flood levels, and the central allowance again when designing safe access.
6. Model and design. Apply that uplift in the hydraulic model, set finished floor levels above the climate-change-adjusted flood level with freeboard, and confirm no worsening of risk elsewhere.

Swap the housing for a substation (essential infrastructure) and the percentile steps up to higher central. Move the site to the coast and you would instead be assessing the higher central and upper end sea level allowances. The method stays the same; only the inputs change. Our walkthrough on how to do a flood risk assessment shows how this fits the wider report.

Credible maximum scenarios, H++ and the adaptive approach

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.

• Sea level: apply the H++ allowance, a total rise of 1.9m to 2100.
• Peak river flow: apply the upper end (95th percentile) allowance.
• Wind and waves: apply the sensitivity test allowances on top of the standard figures.
• Storm surge: add a further 2mm per year from 2017 on top of the sea level allowance.

Running these scenarios does not usually mean designing to the very worst case today. It tests how sensitive the scheme is to a changing climate, which supports an adaptive approach: leaving room to raise defences, add storage or adapt 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 a simple example.

Do the allowances apply in Wales and Scotland?

No. The Environment Agency's climate change allowances apply in England only. Wales and Scotland run their own systems, so a site across the border needs the right national guidance rather than the English figures.

In Wales, Natural Resources Wales sets the approach and flood risk is assessed through a Flood Consequences Assessment under the policy that replaced TAN15, not through the English flood zones and allowances. In Scotland, SEPA publishes its own climate change allowances. If you are working in Wales, our guides to the new TAN15 and to flood consequence assessments set out what is different.

How the allowances fit the wider policy picture

Climate change allowances are a technical input, but they sit inside the planning system. The National Planning Policy Framework and its Planning Practice Guidance require flood risk to be managed over a development's lifetime, taking climate change into account, and the allowances are how that requirement is met in numbers. The framework itself is being rewritten, with a revised NPPF expected in 2026, though the allowances guidance is separate and unaffected by that reform. Our summary of the revised NPPF and flood risk covers what is changing in policy, while this page stays focused on the allowance figures.

Design to the allowances from the start. It is far cheaper to size drainage and set floor levels correctly in the first submission than to retrofit them after an Environment Agency objection.

Frequently asked questions