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When and how local planning authorities, developers and their agents should use climate change allowances in flood risk assessments.

Making allowances for climate change in your flood risk assessment will help minimise vulnerability and provide resilience to flooding and coastal change.

The Environment Agency will check that you have used climate change allowances when they provide advice on flood risk assessments and strategic flood risk assessments.

For guidance on designing flood and coastal risk projects, schemes and strategies see Flood and coastal risk projects, schemes and strategies: climate change allowances.

For guidance on how to prepare a site-specific flood risk assessment see Flood risk assessments if you’re applying for planning permission.

For guidance on strategic flood risk assessments see How to prepare a strategic flood risk assessment.

For guidance on how the planning system will help minimise vulnerability and provide resilience to the impacts of climate change, see the:

Local planning authorities should refer to these when they prepare local plans and consider planning applications.

Climate change allowances are predictions of anticipated change for:

To increase resilience to flooding and coastal change, you should make allowances for climate change in your flood risk assessment.

There are allowances for different climate scenarios over different epochs, or periods of time, over the coming century. They include figures for extreme climate change scenarios.

The Environment Agency uses climate change allowances when they provide advice on flood risk assessments and strategic flood risk assessments.

There may be circumstances where local evidence supports using other data or allowances. For example, the impact of climate change on peak river flow may not be the same for all rivers in a management catchment.

The Environment Agency may want to check how and why you used other data in your plans and proposals.

Peak river flow allowances show the anticipated changes to peak flow by management catchment. Management catchments are sub-catchments of river basin districts.

The range of allowances is based on percentiles. A percentile describes the proportion of possible scenarios that fall below an allowance level. The 50th percentile is the point at which half of the possible scenarios for peak flow fall below it, and half fall above it.

An allowance based on the 50th percentile is exceeded by 50% of the projections in the range. At the 70th percentile it is exceeded by 30%. At the 95th percentile it is exceeded by 5%.

For flood risk assessments and strategic flood risk assessments, the Environment Agency, as a statutory consultee, uses the management catchment climate change allowances from the peak river flow map as benchmarks.

To work out which management catchment allowances to use, you need to:

In some locations the dominant source of flooding will be from a neighbouring management catchment. If so, use the allowances from the neighbouring management catchment to assess the risk for your development or site allocation. Contact the Environment Agency if you are unsure which allowance to use.

The Environment Agency also provide these allowances in the peak river flow climate change allowances by management catchment table – you have to know your management catchment to get the information you need.

You also need to know the flood zone your development is located in.

Use the flood risk vulnerability classification to classify the vulnerability of your development.

Assess both the central and higher central allowances for strategic flood risk assessments.

In flood zones 2 or 3a for:

In flood zone 3b for:

Apply the peak river flow allowances to developments and allocations where the strategic flood risk assessment shows an increased risk of flooding in the future. This includes locations that are currently in flood zone 1, but might be in flood zone 2 or 3 in the future.

To ensure the safety of people using the development when designing safe access, escape routes and places of refuge, use the central allowance for all development types except for essential infrastructure. Use the higher central allowance for this.

If the local planning authority consider the development is appropriate, even though it will not follow the flood zone compatibility categories for flood zones 2, 3a or 3b, use the higher central allowance.

Where it is appropriate to apply a credible maximum scenario, use the upper end allowance.

The appropriate allowance to assess off-site impacts and calculate floodplain storage compensation depends on land uses in affected areas. Use the:

You should also consider likely future land uses shown by local plan allocations or unimplemented extant planning permissions. The Environment Agency will want to see evidence from the developer to prove they have done this.

Contact the Environment Agency if you are unsure which allowance to use for flood storage compensation.

Increased rainfall affects surface water flood risk and how you need to design drainage systems.

The peak rainfall allowances map shows anticipated changes in peak rainfall intensity. Use these for site-scale applications (for example, drainage design), and for surface water flood mapping in small catchments (less than 5 square kilometres) and urbanised drainage catchments. A drainage catchment is urban if the land use is a town or city. If you are unsure if your catchment is urban or rural, please contact the lead local flood authority.

For modelling large areas (larger than 5 square kilometres) with rural land use, direct rainfall modelling is unlikely to be appropriate and fluvial flood risk should be assessed using the peak river flow allowances. Do not use the peak river flow allowances to adjust rainfall totals as they are not compatible.

To work out which management catchment allowances to use, you need to:

The Environment Agency also provides the peak rainfall allowances in the table Peak rainfall climate change allowances by management catchment. You have to know your management catchment to find the information you need.

Use the development lifetime guidance to work out the lifetime of your development. You should consider residential development to have a minimum lifetime of a 100 years.

This includes development proposed in applications or local plan allocations.

For flood risk assessments and strategic flood risk assessments assess the upper end allowances. You must do this for both the 1% and 3.3% annual exceedance probability events for the 2070s epoch (2061 to 2125).

Design your development so that for the upper end allowance in the 1% annual exceedance probability event:

Follow this guidance to help you do this:

For development with a lifetime between 2061 and 2100 take the same approach but use the central allowance for the 2070s epoch (2061 to 2125).

For development with a lifetime up to 2060, take the same approach but use the central allowance for the 2050s epoch (2022 to 2060).

Contact the lead local flood authority if you are unsure which allowance to use.

In some locations the allowance for the 2050s epoch is higher than that for the 2070s epoch. If so, and development has a lifetime beyond 2061, use the higher of the two allowances.

There are a range of allowances for each river basin district and epoch for sea level rise. They are set out in table 1 and are based on percentiles. A percentile describes the proportion of possible scenarios that fall below an allowance level.

An allowance based on the 70th percentile is exceeded by 30% of the projections in the range. At the 95th percentile it is exceeded by 5% of the projections in the range.

For flood risk assessments and strategic flood risk assessments, assess both the higher central and upper end allowances.

The Environment Agency will want to see if you have considered whether it is appropriate to apply the H++ allowances for your flood risk assessment or strategic flood risk assessment. Where applicable you should do H++ allowance assessments as well as assessing the sea level rise allowances in table 1.

The following maps will help you find out which river basin district you are in:

The allowances in table 1 account for slow land movement. This is due to ‘glacial isostatic adjustment’ from the release of pressure at the end of the last ice age. The northern part of the UK is slowly rising and the southern part is slowly sinking. This is why net sea level rise is less for the north-west and north-east than the rest of the country.

To calculate sea level using table 1, add the allowances for the appropriate one of the 6 geographical areas:

Where it is appropriate to apply a credible maximum scenario, use the H++ allowance. There is no H++ value for sea level rise beyond 2100.

For the change to relative mean sea level use the H++ scenario of 1.9m for the total sea level rise to 2100.

To help you decide which allowances to use to address flood risk for a development or development plan allocation, consider the:

The Environment Agency expects sea level rise to increase the rate of coastal erosion. Use the coastal erosion risk maps to plan for any changes in the position of the coastline, together with any designated coastal change management areas and relevant policies in local plans.

Coastal erosion risk maps are based on the best available data. They show the shoreline management plan for each stretch of coast and erosion predictions where there is no policy to maintain defences.

Wave heights may change because of:

Environment Agency coastal models may already include wind speed and wave height allowance. Ask the Environment Agency if they have included wind speed and wave height allowance in your model.

The Environment Agency will want to see if you have used the appropriate allowance for wind speed and wave height in table 2 if these allowances are not included in your coastal model.

Use the epoch that the lifetime of your development falls into to choose the appropriate allowance. If your development lifetime is beyond 2056, use the allowance for the 2056 to 2125 epoch. The allowances are not cumulative across the epochs.

Use the sensitivity test allowances in addition to the offshore wind speed and extreme wave height allowances for assessments where it is appropriate to apply a credible maximum scenario.

The present day extreme sea levels in Coastal design sea levels – coastal flood boundary extreme sea levels (2018) account for storm surge. Most Environment Agency coastal models use these extreme sea levels. Ask the Environment Agency when you get the model.

If your coastal model does not include an allowance for storm surge, or you are creating a new model, use the Coastal design sea levels – coastal flood boundary extreme sea levels (2018) for your extreme sea levels. Then apply the climate change sea level rise values in table 1.

Where it is appropriate to assess a credible maximum scenario, add 2mm for each year starting from 2017.

Nationally significant infrastructure projects (NSIPs) are major infrastructure projects such as new harbours, roads, power stations and power lines. If you develop NSIPs you may need to assess the flood risk from a credible maximum climate change scenario. Check the relevant national policy statement.

In other cases, such as new settlements or significant urban extensions, you may also need to assess the flood risk from a high impact climate change scenario. In these circumstances you should use:

You should treat this as a ‘sensitivity test’. It will help you assess how sensitive your proposal is to changes in the climate for different future scenarios. This will help to ensure your development can be adapted to large-scale climate change over its lifetime.

Some measures to manage flood risk are not necessary now but may be in the future. For example, setting a development away from a river so it is easier to improve flood defences in the future. This is called an adaptive approach.

The Environment Agency will consider whether an adaptive approach is appropriate when they review your plans or proposals.

The Environment Agency can give a free preliminary opinion to applicants who consult us on their proposals outside the statutory consultation process. This will include advice on what allowances to apply and the appropriate approach to incorporating the allowances into assessments.

There is a charge for more detailed pre-application planning advice and reviews of flood risk modelling.

Contact your lead local flood authority through your local council for advice on flood risk from local watercourses (surface or groundwater).

National Customer Contact Centre PO Box 544 Rotherham S60 1BY

Email enquiries@environment-agency.gov.uk

Telephone from outside the UK (Monday to Friday, 8am to 6pm GMT) +44 (0) 114 282 5312

Monday to Friday, 8am to 6pm. The impact of COVID-19 means you may experience some delays in responses.

Clarified in section 'How to use the peak rainfall intensity allowances': surface water flood mapping in small catchments (less than 5 square kilometres) and modelling large areas (larger than 5 square kilometres).

Updated the peak rainfall allowances using UKCP Local 2.2km projections and supporting research FUTURE-DRAINAGE. They are provided by management catchments instead of at a national scale (for England). Peak rainfall allowances are provided for 1% annual exceedance probability (AEP) events and for 3.3% AEP events, and for 2 epochs rather than 3. The guidance on how to apply peak rainfall allowances has changed, using the central allowance for development with a lifetime up to 2100 and the upper end allowance for development with a lifetime from 2100 to 2125.

Edited the section 'Peak river flow allowances': In some locations the dominant source of flooding will be from a neighbouring management catchment. If so, use the allowances from the neighbouring management catchment to assess the risk for your development or site allocation.

There is an error in the 20 July 2021 update. We used UKCP18 projections, not UKCP19 to update the peak river flow allowances.

We have used the UKCP19 projections to update the peak river flow allowances and we have based them on management catchments instead of river basin districts. We have also changed the guidance on how to apply peak river flow allowances. You now use (a) the central allowance for all assessments except for essential infrastructure, where you use the higher central allowance, (b) the upper end for ‘credible maximum scenario’ assessments, and (c) the central allowance to calculate flood storage compensation, except for where essential infrastructure is affected, where you use the higher central allowance.

We have edited the sections on peak river flow, sea level rise, wind speed, wave height and storm surge to include guidance on how to use High ++ allowances in developments where they need to be assessed.

Correction to example 3, to get sea levels: treat subsequent time periods 2066 to 2095 and 2096 to 2125 as you would 2036 to 2065.

We have made the following updates: 1) Updated the sea level rise allowances using UKCP18 projections. 2) Added guidance on how to a) calculate flood storage compensation, b) use peak rainfall allowances to help design drainage systems, c) account for the impact of climate change on storm surge, d) assess and design access and escape routes for less vulnerable development. 3) Changed the guidance on how to apply peak river flow allowances so the approach is the same for both flood zones 2 and 3.

Added: This guidance is being revised in line with the UK Climate Projections 2018. Please contact the Environment Agency for interim guidance if you are preparing a flood risk assessment for a development or local plan affected by tidal flooding.

Temporary exceptions sections removed and information for flood zone 1 added

Epochs in tables 2,3 and 4 updated.

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