Climate Change Allowances for Flood Risk: EA Guidance Explained

What Are Climate Change Allowances?

Climate change allowances are percentage uplifts that must be applied to current peak flood flows, rainfall intensity, sea levels, and wave conditions to account for the projected impacts of climate change over the lifetime of a development. They are published by the Environment Agency and are a mandatory requirement in flood risk assessments and surface water drainage design.

The rationale is straightforward. Climate change is projected to increase the intensity and frequency of rainfall events, raise sea levels, and alter river flow patterns. A development designed to today’s flood levels may not be safe in 50 or 100 years when the climate has changed. The allowances ensure that developments are designed to remain safe and functional throughout their intended lifespan.

For residential development, the design lifetime is typically 100 years, meaning the assessment must look ahead to the 2080s. For commercial development, a shorter horizon of 60 years may be appropriate, using the 2070s or 2080s epoch. The choice of epoch directly affects which allowance percentages are applied.

How the Allowances Work

The EA publishes allowances for four main categories:

1. Peak River Flow Allowances

These are percentage uplifts applied to peak river (fluvial) flow rates. They represent the expected increase in the peak flow of a river during a flood event due to climate change. Higher peak flows mean deeper and more extensive flooding.

Peak river flow allowances are differentiated by:

• River basin district — England is divided into multiple river basin districts (such as Thames, Severn, Humber, Northumbria), each with its own set of allowances reflecting regional climate projections.
• Time period (epoch) — allowances are provided for three future periods: the 2050s (2040-2069), the 2070s (2060-2089), and the 2080s (2070-2115).
• Percentile — allowances are provided at different confidence levels: central (50th percentile), higher central (70th percentile), upper end (90th percentile), and H++ (a credible maximum scenario).

For most residential development, the 2080s epoch is used. The choice of percentile depends on the vulnerability of the development and the flood zone:

• Central estimate is appropriate for less vulnerable development in Flood Zone 2.
• Higher central is commonly used for more vulnerable development in Flood Zone 2.
• Upper end is typically required for more vulnerable development in Flood Zone 3a, and for sensitivity testing in Flood Zone 2.
• H++ is used for sensitivity testing on the most critical developments and essential infrastructure.

2. Peak Rainfall Intensity Allowances

These are percentage uplifts applied to rainfall intensity used in drainage design and surface water flood risk assessment. They account for the projected increase in the intensity of short-duration, high-intensity storms — the type of events that cause surface water flooding and challenge drainage systems.

Current EA guidance provides the following rainfall allowances:

Epoch	Central Estimate	Upper End
2050s (2040-2069)	10%	20%
2070s (2060-2089)	15%	30%
2080s (2070-2115)	20%	40%

These apply nationally and are not differentiated by location. For drainage design, most LLFAs and the EA require the use of the upper end allowance for the appropriate epoch. Some LLFAs specify a fixed percentage (commonly 40% for residential development) in their local guidance.

3. Sea Level Rise Allowances

These are absolute values (in millimetres per year, or cumulative totals) added to current sea levels to account for projected sea level rise. They affect tidal flood risk assessments and the design of coastal defences.

Sea level rise allowances are differentiated by region (the EA provides separate figures for different stretches of the English coastline) and by time period. The current allowances are based on UKCP18 marine projections and include contributions from thermal expansion, ice sheet dynamics, and land movement.

4. Offshore Wind Speed and Wave Height

For developments in coastal locations, allowances for increased offshore wind speed (up to 10%) and wave height (up to 10%) may also need to be applied. These affect the design of coastal defences and the assessment of wave overtopping risk.

Applying Allowances to Flood Risk Assessments

In a flood risk assessment, climate change allowances are applied to the hydraulic model or flood level estimation to determine future flood extents and depths. The process is:

Step 1: Establish the Baseline

Determine the current peak flow or flood level for the design event (typically the 1% annual probability event for fluvial, or 0.5% for tidal). This is usually obtained from EA data (Product 4) or from bespoke flood modelling.

Step 2: Apply the Allowance

Increase the peak flow by the appropriate percentage allowance for the relevant epoch and percentile. For example, if the current 1% annual probability peak flow is 50 m3/s and the 2080s upper end allowance is 65%, the climate change-adjusted flow is 82.5 m3/s.

Step 3: Determine Future Flood Levels

Run the hydraulic model (or apply the allowance to the flood level estimation) with the increased flow to determine the future flood level at the site. This gives the design flood level that finished floor levels and other mitigation measures must be set against.

Step 4: Sensitivity Testing

Best practice — and increasingly a requirement — is to test at multiple allowance percentiles. This shows how sensitive the site is to different climate change scenarios. A site where the difference between central and upper end flood levels is small is less sensitive than one where it causes a significant increase in flood depth or extent.

Sensitivity testing is particularly important for sites on the edge of flood zones. A site currently in Flood Zone 2 may fall within the climate change-adjusted Flood Zone 3 extent under higher allowances, fundamentally changing the planning requirements.

Applying Allowances to Drainage Design

For surface water drainage (SuDS) design, climate change allowances are applied to the design rainfall. The process is:

1. Obtain the design rainfall for the site using FEH rainfall data (typically via the FEH Web Service).
2. Apply the appropriate uplift — for residential development with a 100-year design life, the 2080s upper end allowance (currently 40%) is commonly used.
3. Size the drainage system — attenuation storage, pipe sizes, and SuDS features must be designed to accommodate the uplifted rainfall intensity while restricting discharge to the agreed rate (typically the greenfield runoff rate).
4. Demonstrate no flooding for the 1 in 30 year event and managed flooding for the 1 in 100 year plus climate change event.

The climate change uplift on rainfall significantly increases the required attenuation storage volume. A 40% increase in rainfall intensity can increase the storage requirement by 50% or more, depending on the site characteristics and discharge rate. This has direct implications for site layout, open space provision, and development density.

Choosing the Right Epoch

The epoch determines which set of allowances is used, and the choice depends on the intended lifetime of the development:

• Residential development: 100-year lifetime from the date of the planning application, typically requiring the 2080s epoch.
• Commercial/industrial development: 60-year lifetime is often appropriate, which may allow the use of the 2070s epoch. However, some LPAs and the EA may still require the 2080s epoch depending on the vulnerability of the development.
• Infrastructure: Design life varies. Major infrastructure may have a 100-year or longer design life, requiring the 2080s epoch and potentially the upper end or H++ allowance.
• Temporary development: If genuinely temporary (such as construction compounds with a defined lifespan), shorter epochs may be justified.

The epoch should be agreed with the EA and/or LLFA before the assessment is prepared. Using the wrong epoch is a common cause of EA objections.

Regional Variation

Peak river flow allowances vary significantly by river basin district. This reflects the geographic variation in projected climate change impacts — wetter winters are projected to affect western and northern catchments more than eastern and southern ones.

As an example, the 2080s upper end peak river flow allowance can range from around 50% in drier eastern catchments to over 100% in wetter western and northern catchments. This means that a development in the north-west of England may need to accommodate a doubling of current peak flows, while a similar development in East Anglia may only need to account for a 50% increase.

The regional variation makes it essential to check the specific allowances for the river basin district in which your site is located. Using a generic national figure is not acceptable and will be identified during review.

The EA publishes the allowances on their website, organised by river basin district. Your flood risk consultant should identify and apply the correct figures for your site’s location.

Recent Changes and Future Updates

The EA’s climate change allowances are periodically updated to reflect new climate science and projections. Key recent developments include:

• UKCP18 basis: The current allowances are based on the UK Climate Projections 2018 (UKCP18), which replaced the earlier UKCP09 projections. UKCP18 generally projects higher increases in peak flows, particularly for the upper end and H++ scenarios.
• Regional differentiation of peak flows: Previous guidance used national allowance bands. The current guidance provides allowances at the river basin district level, offering more geographically specific projections.
• Increased rainfall uplifts: The rainfall intensity allowances have increased in recent updates, reflecting growing evidence of intensification of short-duration storms. The 2080s upper end allowance of 40% is higher than the 30% figure used under previous guidance.
• Sea level rise revisions: Sea level rise projections have been revised upward, reflecting updated ice sheet modelling. This affects coastal and tidal flood risk assessments.

When a planning application is submitted, the EA expects the most recent allowances to be used. An FRA prepared using outdated allowances will be challenged. If your FRA was prepared some time ago and you have not yet submitted the planning application, check whether the allowances have been updated in the interim.

Practical Implications for Developers

Climate change allowances are not an abstract policy exercise — they have direct, material impacts on development viability and design:

• Finished floor levels are set relative to the climate change-adjusted flood level. Higher allowances mean higher floor levels, which can affect building heights, access arrangements, and visual impact.
• Floodplain compensation volumes increase when climate change flows are applied, requiring more land to be set aside for flood storage.
• Drainage storage requirements increase substantially with higher rainfall uplifts, affecting the amount of open space needed for SuDS features or the size of underground tanks.
• Development density may need to be reduced on constrained sites where climate change requirements consume a larger proportion of the developable area.
• Viability can be affected where the cost of climate change mitigation (higher floor levels, larger attenuation systems, floodplain compensation) makes a scheme uneconomic.

The key takeaway for developers is to factor climate change into feasibility assessments from day one. A site that appears viable based on current flood levels may become marginal or unviable when the correct climate change allowances are applied. Understanding this early avoids wasted investment in design and planning.

For expert advice on applying climate change allowances to your project, contact our team. Our engineers work with these figures daily and can advise on how they affect your site’s development potential. See also our flood risk assessment and flood modelling services.