Flood Risk Assessment: When You Need One and How to Get It

When a flood risk assessment is required for a planning application - what it involves, who can prepare one, and how flood zones affect your proposal.

Flood risk is one of the most important constraints in the planning system. If your development site is in a flood-prone area - or if your proposal could increase flood risk elsewhere - you will likely need a flood risk assessment (FRA) as part of your planning application. Without one, the council will not validate your application.

What Is a Flood Risk Assessment?

A flood risk assessment is a technical report that evaluates:

• The risk of flooding to the proposed development - from rivers, the sea, surface water, groundwater, and sewers
• The risk of flooding from the proposed development - whether the development would increase flood risk elsewhere (e.g. by increasing impermeable surface area)
• How the development will manage flood risk - through design, layout, drainage, and resilience measures

An FRA is a technical document, typically prepared by a civil engineer or flood risk consultant.

When Do You Need One?

You need a site-specific FRA if:

• The site is in Flood Zone 2 or 3 (medium or high risk of river/sea flooding)
• The site is 1 hectare or larger (even if in Flood Zone 1)
• The site is in an area with known drainage problems or critical drainage areas identified by the council
• The development involves a change of use to a more vulnerable category - e.g. commercial to residential (residential is classed as "more vulnerable" than commercial)
• The council or Environment Agency has specifically requested one

For householder applications (extensions, outbuildings), an FRA is only required if the site is in Flood Zone 2 or 3. A simple statement confirming the property is in Flood Zone 1 is usually sufficient for small projects in low-risk areas.

Flood Zones Explained

The Environment Agency maps all land in England into flood zones based on the probability of river and sea flooding (ignoring the presence of defences):

You can check the flood zone for any location using the Environment Agency's Flood Map for Planning (available on gov.uk). Note that surface water flooding is mapped separately - a site can be in Flood Zone 1 (low river/sea risk) but still have significant surface water flood risk.

The Sequential Test

For developments in Flood Zone 2 or 3, the council must apply the Sequential Test. This asks: could the development be located in an area with a lower flood risk? The applicant must demonstrate that there are no reasonably available alternative sites at lower flood risk.

For individual householder applications, the Sequential Test is usually straightforward - you cannot relocate your house. For new-build residential or commercial development, you may need to search for alternative sites and demonstrate why they are not available or suitable.

The Exception Test

If a development passes the Sequential Test but is still in Flood Zone 3, a more vulnerable use (e.g. residential) must also pass the Exception Test. This requires:

1. The development provides wider sustainability benefits to the community that outweigh the flood risk
2. A site-specific FRA demonstrates the development will be safe for its lifetime (taking climate change into account), will not increase flood risk elsewhere, and where possible will reduce overall flood risk

What an FRA Contains

A site-specific FRA typically includes:

• Site description - location, topography, geology, existing drainage
• Flood risk sources - assessment of risk from rivers, sea, surface water, groundwater, sewers, and artificial sources
• Climate change allowances - projections of how flood risk will increase over the lifetime of the development
• Sequential and Exception Test - where applicable
• Mitigation measures - finished floor levels, flood resilient construction, safe access and egress, flood warning and evacuation plans
• Surface water drainage strategy - how the development will manage surface water to avoid increasing flood risk (SuDS - sustainable drainage systems)

SuDS (Sustainable Drainage Systems)

Almost all new development is expected to incorporate SuDS to manage surface water. Common SuDS features include:

• Permeable paving for driveways and parking areas
• Rainwater harvesting (water butts, underground storage tanks)
• Swales (shallow grass-lined channels)
• Rain gardens (planted areas designed to absorb runoff)
• Green roofs
• Attenuation ponds or tanks (storing water and releasing it slowly)

The principle is that surface water runoff from the developed site should be no greater than from the undeveloped site - and ideally should be reduced.

Costs

Tips

1. Check the flood zone early - before buying a site or designing a scheme, check the Environment Agency flood maps
2. Pre-application advice - for sites in Flood Zone 2 or 3, seek pre-application advice from both the council and the Environment Agency
3. Design for resilience - raising floor levels, using flood-resistant materials, and avoiding habitable rooms at ground level can make development in flood-prone areas acceptable
4. Don't forget surface water - surface water flooding affects many sites in Flood Zone 1. Check the Environment Agency's surface water flood maps as well as the river/sea flood maps