Flood Zones

Paul Vernon

Paul Vernon · Surveyor (retired)

Surface water flooding happens due to heavy rainfall when drains and watercourses are not able to cope with the amount of water run-off from buildings, footpaths and roadways.

If you are observant, when you drive around the countryside, you may notice that some old houses are built on slight rises in the ground, or inexplicable positions. The old folk when they were building houses and farm buildings were able to draw on generations of local knowledge to avoid areas that they knew to be risky. For example, there is a farm complex in the floodplain near where I live which is never inundated when the river floods, as it does with great regularity. The waters rise until they reach the farmyard, but never enter it, because there is an almost imperceptible rise in the ground level. The old folks were observant!

Nowadays, it’s unusual for residential development to be driven by people with such local knowledge and so without this expertise it is possible for buildings to be built in locations that are at risk of being affected by flooding. Especially if flooding occurs rarely or the property is some distance from a watercourse.

The Problem

People often assume that distance from a watercourse equals immunity, but when the terrain is flat, flood water can extend over very large areas.

It’s not only flat areas that are at risk. When significant volumes of water move down valleys and culverts, if they meet an obstacle, water can back up and cause problems where you might not expect it. Such an obstacle may be where a bridge or culvert is obstructed by debris, or the space under the bridge or culvert isn’t big enough to accommodate the unusual volumes of water that it encounters. Or it might be where two water courses meet and the volume of water suddenly increases.

Information Sources

A great deal of research has taken place in recent years into the nature, frequency and extent of flooding and each of the national environment agencies publish flood maps which are easily accessible online. These show clearly the areas where flooding might occur, and also the frequency and impact.

So if I type in an example address in Edinburgh into the Scottish Environmental Protection Agency’s flood website (SEPA) then it tells me that it is a Low Risk area. It then explains that this means gives the likelihood of surface water flooding within a 50 metre radius of this location and that Low Risk means that each year this area has a 0.1% chance of flooding. You can do the same for Wales, England and Northern Ireland through this website. These can provide a surprising level of detail and accuracy. It’s important to note that these do not take into account the effect of any flood defences, which can be incredibly effective in protecting properties if well designed and maintained.

What research can you do?

Well, obviously you’ll check out the websites that I’ve mentioned above, but it’s not always that simple. You would think that developers and Planning Authorities would also reference these resources and ensure that developments don’t take place in flood zones. If only.

Planners and developers have a responsibility to ensure future developments are sustainable and do not increase flood risk to the site or surrounding area. This is steered by national and local policy, and developers are required to consider all types of flooding and use sustainable drainage systems to manage surface water. However, figures show that more than 2,000 houses were granted planning permission in flood risk areas in Wales between 2016 and 2019. The same article says that while applications to build 2,159 homes on flood plains were granted by councils in Wales over those three years, plans for just 246 were rejected.

In their defence, developers are often constrained by the land that they have available, and will usually try to make a case why their scheme should be allowed to proceed even if some of the evidence might suggest otherwise. When any Planning Application is made the Planners will seek opinion from a variety of experts, known as Statutory Consultees, including SEPA (Scotland), the Environment Agency (England), Natural Resources Wales or the Northern Ireland Environment Agency.

However, you cannot assume that if a development has consent, then it is free from flood risk. It’s quite possible that in granting consent, the planners will have allowed for the effectiveness or construction of flood defences, but the evidence shows that sometimes they, or even the government, has overruled the concerns of their Statutory Consultees with little or no mitigation required.

What this means is that you have to look out for yourself. I make no apologies for repeating myself. You should carefully check the flood zone websites. If you are in a flood zone, and if possible (and these days it’s usually easy enough online) I also would search the planning records to understand why the development was allowed to proceed if the property is in a flood zone.

You should ensure that your solicitor gets an adequate response from the seller’s questionnaire on whether there has been flooding. Thinking outside the box, you could get a quote for buildings insurance. Insurance companies have good records and if there have been claims, you’ll find out by way of an expensive quote, or possibly even a refusal to quote! You could even have a good look around the neighbourhood. I know that you will have done that already, but are there signs of regular flooding, such as flood barriers, sandbags, tidemarks on buildings or similar?

What can you do if you already own the house?

Despite all the gloom, there are things that you can do to ameliorate the risk and effects, but it requires thought.

As mentioned, floods occur for lots of reasons, whether it’s storm surges, river floods, surface or groundwater flooding, or reservoir or dam failures. You might think of floodwater as being overflowing river water coming through the doors (or windows if it’s deep enough), but it could just as easily come up through the drains and sewers. You could have the most perfect barrier around the house, only to find water coming up through your drains and toilet, not a pleasant thought! Take advice from a surveyor, because there really is no point in fitting a flood barrier if your risk of flooding is coming from underneath.

Once you have determined where the risk comes from you can then develop a list of resilience options. This could be either keeping the water out (dry proofing), or making the subsequent clean up easier (mitigation), or a combination of both.

include dry proofing measures which are designed to resist floodwater, and wet proofing features tailored to minimise the time and cost of recovery if the house does flood.

Dry proofing typically involves fitting temporary flood barriers, flood doors, hinged barriers, rolling door systems or flood-resistant windows, repointing bricks and repairing mortar, which improves resistance to water. You can also add seals to doors and windows, fit automatic or manual air brick covers and install non-return valves. The average flood in this country is about 0.3 metres high, so it’s perfectly possible to keep that kind of water out of your home in the first place. You should however, seek professional advice. Houses are not normally designed to hold back water, and at above about 0.6 metres of water, the loading on a building becomes substantial and it’s much safer to let water inside than for it to build up outside and affect the structural integrity of the building.

Mitigation involves adapting the building’s fabric and interiors to incur minimal damage during a flood and to allow for speedy repairs when floodwaters recede. It can involve elevating sockets, or feeding the wiring from the first floor down, rather than from the ground up. It could mean changing the fabric of the building from loose cell insulation to closed cell insulation, which doesn’t absorb as much water. You can install water-resilient plasterboard, apply lime plaster, which can remain on the wall and dries out naturally, have loose fitting carpets which can be rolled up easily, and lay specially-treated floor coverings. The biggest price tag when recovering from a flood is usually the kitchen. An MDF kitchen may be cheaper but it will disintegrate in a flood. Replace it with solid wood or powder-coated stainless steel, which is very retro-looking and beautiful. You can literally wash it down, sanitise, and carry on living there. I have seen cases on TV where householders have got out the pressure washer and been back in the house with a few days or even hours of waters receding, but this takes planning and preparation.

A third, more radical strategy is to take mitigation to another level and reduce exposure to flooding, by repositioning the habitable spaces within a home or raising the building up off the ground.

There’s lots of advice available on the internet, and whilst it does cost, the long term financial (and psychological) savings could be substantial.

Much of these last few paragraphs rely on an article in the Daily Telegraph in 2020.

Would I buy a house in such an area?

To be honest, I’m not sure. I’d think very carefully before I did so, but it isn’t as black and white as you might think. You only have to read the papers to realise that even a short period of flooding can take many months to remedy, with householders having to live in rented accommodation and even after they return home being unable to sleep soundly when it is raining heavily. On the other hand, some people want to live near a river, and are prepared to accept the consequences, perhaps by adapting their houses so that they can be quickly and easily washed out and made ready for living in again.

You have to make your own decision, but as with all things in life, just make sure that you are in possession of as much information as possible and take whatever technical advice you feel is appropriate.


What I have written here refers to what I would term mass volume flooding, the risk of which can be predicted by computer models. But I am aware of a property that is nowhere near any flood zone, yet during a severe rainstorm it had an influx of water. The driveway had been laid in such a way as to direct most of the rainwater towards the house, and then an ornamental wall allowed the water to build up so that it overflowed into the house. A simple realignment of the driveway has been sufficient to permanently remove this risk. So wherever your prospective home is, just have a good look at the house and its surroundings.