Flood Risk Assessment for House Extensions

House extensions are one of the most common types of planning application in the UK, and for many homeowners, the first time they encounter a Flood Risk Assessment (FRA) requirement is when they apply for planning permission for an extension to their home.

The requirement can come as a surprise, particularly for homeowners who have lived in their property for years without experiencing any flooding. But the planning system takes a precautionary approach to flood risk, and the rules apply regardless of whether the property has actually flooded.

This article explains when an FRA is needed for a house extension, what it should cover, and how to navigate the process efficiently.

When Is an FRA Required?

The National Planning Policy Framework (NPPF) Requirement

The NPPF and its associated Planning Practice Guidance (PPG) set out when a Flood Risk Assessment is required. For householder applications (including extensions), an FRA is required in the following circumstances:

The site is in Flood Zone 2 or 3. If any part of the application site falls within Flood Zone 2 (medium probability) or Flood Zone 3 (high probability), a site-specific FRA is required regardless of the size of the extension.

The site is in Flood Zone 1 but the development area is 1 hectare or more. This threshold is rarely relevant for house extensions, as most extensions are well below 1 hectare. However, for very large properties with extensive grounds, it could apply.

The site is in a designated Critical Drainage Area. Some local authorities have designated Critical Drainage Areas (CDAs) through their Strategic Flood Risk Assessments. If the site is in a CDA, an FRA may be required even in Flood Zone 1.

There is known flood risk from other sources. Even in Flood Zone 1, the LPA may request an FRA if there is evidence of surface water flood risk, groundwater flood risk, or historical flooding at or near the site. The PPG makes clear that flood risk from all sources should be considered.

Local Planning Authority Requirements

Individual local planning authorities may have their own requirements for FRAs that go beyond the national policy. Some LPAs require FRAs for any householder application in a flood zone, regardless of the extension size. Others may require a drainage assessment or surface water management plan even where a full FRA is not needed.

Check the LPA’s validation checklist before submitting your application. This document lists all the information required to make the application valid, including any FRA requirements.

The Environment Agency’s Standing Advice

For minor development in Flood Zones 2 and 3 — which includes householder extensions — the Environment Agency does not typically provide bespoke consultation responses. Instead, it publishes Standing Advice that sets out the minimum requirements for FRAs and the design standards that developments should meet.

The LPA uses this Standing Advice to assess the application. This means the FRA needs to address all the points in the Standing Advice, as the LPA will check the FRA against it.

What Should the FRA Cover?

For a house extension, the FRA does not need to be as extensive as one for a major development, but it must address certain key points:

Sources of Flood Risk

The FRA should assess flood risk from all relevant sources:

• Fluvial (river) flooding — using Environment Agency flood zone maps and any available modelling data
• Tidal (sea) flooding — if relevant to the location
• Surface water (pluvial) flooding — using the Risk of Flooding from Surface Water maps
• Groundwater flooding — using British Geological Survey data and local records
• Sewer flooding — using water company records (DG5 register)
• Artificial sources — including reservoirs, canals, and other water bodies

Flood Levels and Climate Change

For extensions in Flood Zone 2 or 3, the FRA should establish:

• The current design flood level (typically the 1% annual probability flood level for river flooding, or the 0.5% level for tidal flooding)
• The climate change adjusted flood level, using the Environment Agency’s published climate change allowances
• How these levels relate to the proposed extension’s floor level and the existing property’s floor levels

The Sequential Approach

The PPG requires a sequential approach to flood risk within the site. For house extensions, this means:

• Locating the extension in the lowest risk part of the site where possible
• Designing the extension so that living accommodation is above the design flood level
• Avoiding placing bedrooms or other sleeping accommodation at ground floor level where the ground floor is below the flood level

Finished Floor Levels

The FRA should specify the finished floor level (FFL) of the extension. The Environment Agency’s Standing Advice requires that:

• Finished floor levels should be set at least 300mm above the 1% annual probability flood level (with climate change allowance) for river flooding
• For tidal flooding, FFLs should be at least 300mm above the 0.5% annual probability flood level (with climate change allowance)
• Where this is not achievable (for example, where the extension must be at the same level as the existing ground floor), the FRA should explain why and set out compensatory measures

Flood Resilience and Resistance

Where the extension’s floor level cannot be raised above the design flood level, the FRA should recommend flood resilience and resistance measures:

• Flood resistance — measures to prevent water entering the extension (flood barriers, waterproof construction below the flood level)
• Flood resilience — measures to reduce damage if water does enter (waterproof plaster, raised electrics, resilient flooring)

Safe Access and Egress

The FRA should consider whether safe access and egress can be maintained during a flood event. This is particularly important for extensions that create additional habitable space, as there may be a need to evacuate the property during a flood.

The EA’s Standing Advice states that safe access and egress should be available during a 1% annual probability flood event (with climate change allowance) for river flooding, or a 0.5% event for tidal flooding. “Safe” is generally defined as a maximum depth of 600mm with a maximum velocity such that the depth multiplied by velocity does not exceed 0.5 m2/s.

Surface Water Drainage

The extension will increase the impermeable area of the site, and the FRA should address the impact on surface water drainage:

• How will rainwater from the extension roof be managed?
• Is there capacity in the existing drainage system?
• Can sustainable drainage systems (SuDS) be used — such as soakaways, permeable paving, or rain gardens?
• What is the impact on surface water flood risk to surrounding properties?

For most house extensions, a simple drainage strategy will suffice. But the FRA should demonstrate that the extension will not increase flood risk elsewhere, which is a fundamental requirement of the NPPF.

The Sequential Test for Extensions

One area of frequent confusion is whether the Sequential Test applies to house extensions.

The Sequential Test is the process by which local planning authorities assess whether a proposed development could be located in an area at lower risk of flooding. For new-build development in Flood Zone 2 or 3, the Sequential Test is a mandatory part of the planning process.

However, for householder applications — including extensions — the Sequential Test does not normally apply. The PPG recognises that homeowners cannot relocate their existing property to a lower-risk area. The sequential approach is instead applied within the site, by locating the extension in the lowest-risk part of the site where possible.

Similarly, the Exception Test does not normally apply to householder extensions. The Exception Test is required for certain types of development in Flood Zones 2 and 3 (particularly “more vulnerable” development in Flood Zone 3a), but householder extensions are generally treated as a continuation of the existing use.

Common Scenarios

Ground Floor Rear Extension

The most common type of house extension. If the existing ground floor is below the design flood level, the extension floor will typically be at the same level (to maintain a level threshold). The FRA should recommend flood resistance and resilience measures for the extension.

Two-Storey Rear Extension

A two-storey extension adds habitable space at both ground and first floor level. The FRA should consider the safety implications of having additional ground-floor accommodation below the flood level, and may recommend that bedrooms are located at first floor level.

Side Extension

Side extensions are often constrained by the available space between the existing property and the site boundary. The FRA should consider whether the extension affects any overland flow routes — if surface water currently flows through the side passage, blocking it with an extension could increase flood risk to neighbouring properties.

Loft Conversion with Dormer

Loft conversions that involve a dormer (and therefore require planning permission) are generally the least problematic from a flood risk perspective, as the additional habitable space is above the flood level. However, the FRA may need to address the additional roof drainage.

Basement Extension

Basement extensions in flood-risk areas are among the most challenging. The PPG states that sleeping accommodation should not be located in a basement that is below the design flood level, due to the risk of rapid inundation and difficulty of escape. An FRA for a basement extension in a flood zone will need to address waterproofing, pumping, safe access and egress, and the risk of groundwater and sewer flooding.

The FRA Process for Extensions

Step 1: Pre-Application Check

Before commissioning an FRA, check whether one is actually required. The Environment Agency’s flood maps (available on GOV.UK) will show the flood zone designation. The LPA’s validation checklist will confirm the local requirements.

Step 2: Data Collection

The FRA consultant will gather data from multiple sources:

• Environment Agency flood zone maps and Product 4 data (detailed flood data package)
• Surface water flood risk maps
• British Geological Survey groundwater data
• Water company sewer flooding records
• Local authority SFRA and any relevant flood studies
• Historic flood event records

Step 3: Assessment and Report

The consultant will assess the data, determine the flood risk to the extension, and prepare a report that addresses all the requirements of the NPPF, PPG, and EA Standing Advice. The report will include recommendations for finished floor levels, flood resilience measures, and surface water drainage.

Step 4: Planning Submission

The FRA is submitted as part of the planning application. The LPA will review it against the EA’s Standing Advice and may consult the LLFA on surface water drainage matters.

Step 5: Conditions

If planning permission is granted, it may include conditions relating to flood risk — for example, requiring the implementation of specific flood resilience measures, or requiring a detailed surface water drainage scheme to be approved before construction begins.

Cost and Timescale

For a house extension, a Flood Risk Assessment typically costs between GBP 500 and GBP 1,200, depending on the complexity of the site and the level of detail required. A straightforward FRA for an extension in Flood Zone 2 will be at the lower end; a more complex assessment for a site in Flood Zone 3 with multiple flood sources will cost more.

The timescale is typically 2-4 weeks from instruction to delivery of the completed report, although this can vary depending on the availability of Environment Agency data (particularly Product 4 data, which can take 20 working days to obtain).

Common Mistakes to Avoid

Not checking the flood zone before starting the design. If the extension needs to accommodate flood resilience measures or raised floor levels, this should be factored into the design from the outset, not retrofitted after the FRA identifies the requirement.

Assuming Flood Zone 1 means no FRA is needed. If there is surface water or groundwater flood risk, the LPA may still require an FRA even in Flood Zone 1.

Ignoring surface water drainage. Even a small extension adds impermeable area. Demonstrating that surface water is managed appropriately is essential for planning approval.

Submitting a generic FRA. The FRA must be site-specific. A generic template that does not address the specific flood risk to the site will be rejected by the LPA.

Not addressing climate change. The FRA must consider how flood risk will change over the development’s lifetime, using the EA’s published climate change allowances.

How Aegaea Can Help

Aegaea prepares Flood Risk Assessments for house extensions across England, Scotland, and Wales. We handle the entire process — from initial data gathering and Environment Agency data requests through to the completed FRA report ready for planning submission.

Our FRAs are prepared by Chartered engineers and are tailored to the specific requirements of each site and local planning authority. We also provide drainage strategies and surface water management plans where required.

If you need an FRA for a house extension, contact us for a quote. We typically turn around FRAs for extensions within 2-3 weeks of receiving all necessary data.