New climate change allowances

Lead Local Flood Authority (LLFA) guidance

Updated climate change allowances were published by the Environment Agency on Gov.uk on 10 May 2022 to reflect the latest projections in UKCP Local (2.2km) and subsequent research ‘Future-drainage: Ensemble climate change rainfall estimates for sustainable drainage’.

This briefing note is aimed at local planning authorities, developers, consultants and flood risk management authorities. It sets out:

• how the new climate change allowances impact on the assessment of surface water flood risk and the design of sustainable drainage systems and;
• explains how, as a Lead Local Flood Authority, we will incorporate these changes to our advice and recommendations as a statutory consultee for major development with surface water drainage.

New climate change allowances and what are they used for

You can find the new climate change allowances on Gov.uk

The guidance on climate change allowances sets out when and how local planning authorities, developers and their agents should incorporate an allowance for climate change in flood risk assessments and in the design of sustainable drainage systems

Who will be affected by the changes

Changes to climate change allowances will need to be incorporated by:

• local planning authorities preparing strategic flood risk assessments
• developer or consultants preparing flood risk assessments for planning applications, designing sustainable drainage systems and development consent orders for nationally significant infrastructure projects
• a risk management authority and other organisations (e.g. Rivers Trusts, RSPB etc) developing a flood and coastal risk project, scheme or strategy

Changes in the updated climate change allowances in surface water terms

Peak rainfall intensity allowance provides is applicable to surface water flood risk and how sustainable drainage systems are to be designed.

Previous allowances

The previous climate change allowances required a peak rainfall intensity allowance of between 20% and 40% to be applied across England, depending only on the lifetime of the development.

The LLFA also required a 10% urban creep allowance to be incorporated into sustainable drainage design, unless evidenced by the developer that urban creep is not applicable.

New allowances

When determining what allowance to apply under the new climate change allowances, two variables must be considered:

1. Location and size of the drainage catchment
   • Urban/Small (flashy) Catchments: Use the peak rainfall allowances mapfor small catchments (less than 5 sq km), or urbanised drainage catchments. A drainage catchment is urban if the land use is a town or city.
   • Rural/Large Catchments: For large rural drainage catchments use the peak river flow allowances.

2. Lifetime of the development
   • For all residential development, the development lifetime is 100 years.
   • For non-residential development, please refer to the 'Flood Risk and Coastal Change' section of the Planning Practice Guidance.

For flood risk assessments and strategic flood risk assessments the upper end allowance must be assessed for both the 1% and 3.3% annual exceedance probability events for the 2070s epoch (2061 to 2125).

The LLFA also requires a 10% urban creep allowance to be incorporated into sustainable drainage design, unless evidenced by the developer that urban creep is not applicable.

Why these changes have been made

These changes have been made because:

• UKCP Local (2.2km) and supporting research Future-drainage provides updated peak rainfall projections that are based on significantly more robust climate modelling than the peak rainfall allowances published in 2016.
• Using management catchments to provide the allowances mean they better reflect the geographic variability in how climate change is likely to affect peak rainfall events across England.
• The 1% and the 3.3% annual exceedance probability events are important for assessing surface water flood risk and informing sustainable drainage system design, in line with the Technical Standards for Sustainable Drainage Systems.
• UKCP Local (2.2km) and supporting research Future-drainage only cover 2 epochs.
• The central allowance, based on the 50th percentile of the high emission scenario (RCP8.5), represents scenario that will meet or exceed a 4°C increase in global mean temperature by 2100. However, UKCP Local (2.2km) only provides projections up to 2080. For development with a lifetime up to 2100 it is robust to use the central allowance, however, for longer life development (such as residential which also is more vulnerable), it is more robust to use the upper end in order to account for the relatively short time horizon in the underpinning projections.

Preparing flood risk assessments and designing sustainable drainage systems

To use the correct climate change allowances for peak rainfall you will first need to identify the management catchment for your assessment. If you do not know the name of your management catchment, use the guidance and link to the peak rainfall allowances map.

For flood risk assessments and strategic flood risk assessments the upper end allowance must be assessed for both the 1% and 3.3% annual exceedance probability events for the 2070s epoch (2061 to 2125).

Development is to be designed so that for the upper end allowance in the 1% annual exceedance probability event:

• there is no increase in flood risk elsewhere, and;
• the development will be safe from surface water flooding

Incorporating these changes

We have incorporated these changes within our internal processes and our advice that we provide to developers, local planning authorities and flood risk management authorities. This includes:

• Pre-application advice service for developers and consultants
• Technical advice provided during the planning process to planners, developers and consultants.

We will also be working to incorporate these changes as soon as possible within:

• the County Council's LLFA Standing Advice for Minor Applications issued to our local planning authorities (currently being updated)
• Sustainable Drainage (SuDS) Proforma. We will work closely with the North West Regional Flood and Coastal Committee to ensure changes are appropriately embedded within the Sustainable Drainage Pro-forma. The updated 2022 Pro-forma will be published on The Flood Hub.

Climate change allowance during the transition period

The updated guidance came into immediate effect from 10th May 2022.

However, as with any national changes, we recognise there will be a transition period whilst we all adjust to the new allowances for climate change. Our approach to plans and major planning applications is outlined below for clarity.

Major planning applications

• Applications submitted before 10 May 2022

Planning applications validated before 10 May 2022 will be processed in line with the previous climate change allowances.

• Applications submitted on or after 10 May 2022

The Lead Local Flood Authority expects all major planning applications validated on or after 10 May 2022 incorporate the new climate change allowances, unless the local planning authority advise otherwise.  

Plan making (development plan documents)

To avoid significant delays to development plans (e.g. local plans) and strategic flood risk assessments that are well progressed or have already been submitted for examination on or before 10 May 2022, the Lead Local Flood Authority will base its advice on the previous guidance in these cases.

Actions for you

If you are a local planning authority, we recommend you consider:

Development management

• Updating your advice/validation requirements for flood risk assessment and sustainable drainage systems in relation to climate change allowances.
• Communicate these changes to any developers who may be affected by them.

Planning policy

• Ensure your Strategic Flood Risk Assessment (SFRA) incorporates the new climate change allowances.
• If your SFRA is in progress, that you contact your consultant to make them aware that these changes need to be incorporated at the earliest opportunity.

If you are a developer or consultant, we recommend you:

• Check with the local planning authority when your planning application was validated and whether they expect you to incorporate the new climate change allowances.
• Update your site-specific flood risk assessment to incorporate the new climate change allowances.
• Revise your sustainable drainage design to incorporate the correct allowance for climate change under the new allowances

If you are a flood risk management authority or other organisation developing a flood and coastal risk project, scheme or strategy, we recommend you:

• Ensure the design of your project or scheme incorporates the correct climate change allowance, in line with the new guidance.
• Ensure any strategy refers to and appropriately incorporates the new climate change allowances.

Contacts

If you require further advice or guidance on how to apply the new climate change allowances to surface water aspects of your flood risk assessment or sustainable drainage design, please contact the Lead Local Flood Authority via suds@lancashire.gov.uk.

If you require further advice or guidance on how to apply climate change allowances to fluvial or coastal aspects in your flood risk assessment, please contact the Environment Agency at clplanning@environment-agency.gov.uk.