Flood Risk Assessments for Residential Developments

Key Points to Keep in Mind

• When will you need to do a flood risk assessment for a residential development

• How do flood zones compare to surface water flooding risk

• What planning permission triggers are there for housing schemes

• What are the usual expectations for mitigating flood risk in homes and gardens

• What does a drainage strategy have to include for residential plots

Flood Risk Assessment Requirements for Residential Developments

There are a few common scenarios that will usually trigger an assessment, and they include things like:

• Building new homes on green or brownfield land\

• Extending or converting properties that are in Flood Zones 2 or 3\

• Building basements that go below ground level\

• Smaller sites that are particularly vulnerable to surface water flooding\

• Changing the use of a building in a way that makes it more vulnerable to flooding

Flood Zones and Residential Planning Risk

• Flood Zone 1 – you're unlikely to get flooded, but you might still need an assessment if surface water is a problem

• Flood Zone 2 – you're at reasonable risk of flooding; you'll almost certainly need to do a flood risk assessment to justify any new housing proposals

• Flood Zone 3 – you're in a part of the country that is very much at risk of serious flooding; planning restrictions on housing development can be pretty tight

When we're talking about flood zones, we're usually looking at information from the Environment Agency Flood Map for Planning.

Planning Permission and Regulatory Requirements

Regulators to watch out for include:

• your local planning authority\

• the Lead Local Flood Authority (LLFA) – they're responsible for dealing with surface water and drainage issues\

• the Environment Agency – they get involved when development might affect a main river or one of the higher-risk flood zones

The sequential test tries to push housing development towards areas that are at the lowest risk of flooding. But if you do want to build in a higher-risk area, the exception test might come into play.

Residential Scenarios and Typical Flood Risk Evidence

• Building a single new home – you'll need to focus on where the flood risk comes from and how to deal with it, particularly if you're building at a low level or near a watercourse

• Home extensions – the flood risk here is often going to be about how water can flow into the property, and how you can keep it out

• Basement conversions – the main worry is usually going to be about the ground water and making sure that the basement is properly waterproofed

• Small housing sites – these can be particularly vulnerable to flooding because they tend to have a lot of impervious surfaces that can make a big difference to how much water runs off the site

Doing Something About Flood Risk in Residential Developments

So what can you do to reduce the risk of flood damage in residential developments? Here are a few ideas:

1. Build your homes in such a way that the flood water can't get in\

2. Make sure that the doors and windows are at a higher level than the predicted flood water\

3. Use materials in the building that can withstand flood water\

4. Give people a safe way to get in and out of the property when the water is high\

5. Build in some local storage to help manage the flow of water when it rains

Flood resistance stops water getting in in the first place. Flood resilience helps to reduce the damage and get the place back to normal as quickly as possible.