Flood Zones - What you need to know

If you're a professional land buyer or self-builder, make sure you understand what Flood Zones can mean to your investment.

Forms of flood risk

The term flood risk is a combination of the probability and the potential consequences of flooding.

The three most common forms of flooding are:
- Fluvial Floods: When the water level in a river, lake or stream overflows onto neighbouring land
- Pluvial Floods: Heavy rain causes flooding independent of any body of water
- Coastal floods: When coastal areas are flooded by the sea, also known as 'storm surge'

Flood zones and development 

Definition of Flood Zone(s)
Flood Zone 1
Areas within Flood Zone 1 are deemed to have less than a 0.1% annual probability of river flooding or flooding from the sea.

Flood Zone 2
Areas within Flood Zone 2 have a medium probability of flooding. The annual probability of flooding from rivers of between 1% and 0.1%, and an annual probability of sea flooding of between 0.5% and 0.1%.

Flood Zone 3 (split into two flood zones: 3a and 3b)
Areas within Flood Zone 3a are deemed to have a 1% or greater annual probability of flooding from rivers and a 0.5% or greater probability of flooding from the sea.

Flood Zone 3b
Flood Zone 3b refers to any land with a 3.3% or higher chance of flooding regardless of the presence of flood defences.

The good news is that although these areas have a higher risk of flooding, it doesn’t always mean they will flood. Plus, the fact that flooding is normally seasonal, means it makes planning ahead of time possible and ideal locations for development. Planning authorities know the chance of flooding, too: developments on flood zones may be granted subject to flood defence works and suitable mitigation.

“Despite warnings from the Environment Agency and the Committee on Climate Change (CCC) between 2001 and 2011, 200,000 new houses were built on the floodplain whilst, at present, there are almost 500,000 homes that have been given planning permission and are waiting to be built on the flood plain.” (Today's Conveyancer)

Flood zones and planning permission 

Though Local Planning Authorities, in line with the NPPF, would prefer not to have houses built on flood plains, the need for housing is universally recognised. Proposals in Flood Zone 1 are most likely to be approved, Flood Zone 3 the least. If you're planning on building in Flood Zones 2 or 3 (a medium or high-risk area), you'll have to do a Flood Risk Assessment at first instance. Your plan will then have to pass the 'Exception Test' - showing the potential impacts on the development if flooding occurs, that building won't increase the chance of flooding, and if there are any flood risk management measures installed to protect the house from any future flood events, to name a few things.

Understanding Flood Maps for Planning Risk 

Although there are risks when it comes to buying land with flood risks and making planning applications, one in six homes are located in areas where there is a significant risk of flooding.

Our Flood Zone land consideration gives you a detailed assessment for planning any development. Knowing the flood risk in an area will be crucial to create resilience against flooding as well as how to reduce the impact of your land flooding.

What to consider when self-building in a flood zone

It is possible to build a new home safely in a flood zone. Houses can be proofed against a flood event, and the lengths you go to will likely depend on the annual probability of flooding. Designing a flood-proofed house breaks down into two goals: one, to prevent water from entering the house in the first place and two, to minimise any lasting damage from water flows if they do get in. Building your ground floor above the potential water level is, in theory, the most logical flood protection measure. This can be difficult to implement in practice, though, and English Councils may be reluctant to grant permission for houses much higher than those in the neighbourhood. 'Tanking' is a waterproof layer installed within walls and floors to prevent flood water from seeping in, and is a common means of flood prevention.    

You can also add several design features to mitigate the damage if water does get in, from adjustable seals on windows and doors to non-return valves in plumbing to prevent sewage from being released in cases of flooding.

Climate change and flood zones 

Flood zones are changing due to climate change. You can find information on the government website about how these changes to our climate affect flood risk in the area:

“The flood zones do not take account of the possible impacts of climate change and consequent changes in the future probability of flooding. Reference should therefore also be made to the Strategic Flood Risk Assessment when considering location and potential future flood risks to developments and land uses. The EA made revisions in March 2016 to the peak flow rate of rivers in various catchments across the country based on climate change predictions. This guidance is available here.”