Does your building plot have a potential flooding risk?

If so, should this call into question creating a basement as part of yourself build project? The answer to this is not simply that basements are not possible. Should your property be located in an area with rising water tables, poor ground conditions or flood risk, it is a possibility, providing you plan correctly. In some cases where the building plot is uneven, a basement is the ideal solution to create a level platform for your house.

However, it is important to recognise there are three flood zones defined by the EA (Environment Agency). These are Flood Zones 1, 2 and 3, Zone 3 being the most likely to flood. Planning applications’ approval is affected by risk. If you’re planning on building in Flood Zones 2 or 3, a Flood Risk Assessment is required. Your application must pass the ‘Exception Test’ – showing that the development will be safe if flooding occurs, that building won’t increase the chance of flooding, and elements are installed to protect the house. Risk comes from two sources – coastal areas affected by tidal flow and risk of a tidal surge, and areas affected by surface water flooding – rivers and ground water. These building areas represent some of the most desirable places to live, but their location comes with a risk which must be mitigated against if possible. Changes in weather patterns and rising sea levels have made this a factor that must be considered for self-builders who want to future proof their houses.

Here are some key points which should be considered if this is a characteristic of your building plot. The primary challenges of building in areas that have a potential risk of high water levels, are:

• Penetration of ground water through the walls of the basement. A traditionally built basement with an internal water management system is not practical in these building conditions, as the structure must be watertight.
• Water levels reaching above the ground floor level and water going into the basement through the stairwell.
• The risk of hydrostatic uplift, meaning if this is not calculated properly and the foundations are not installed correctly, the basement and house can physically float from its original foundation settings just as a boat would in rising water.

Points to check in advance of preparing to build where water could be an issue:

• Ground Investigation Survey including flood risk research.
• Groundwork Planning.
• Drainage Planning.
• If piling is required, engage a Piling Engineer and Piling Contractor.
• Possibly employ the services of a Planning Consultant.

It is important to study the soil survey and ground investigation report for the area to know the details of any existing ground water table levels and likely increases in the water table. Fortunately, this information is freely available from the local planning office. Another consideration is the level of exterior insulation required. Even in areas with high water levels, insulation will still function and improve the heat retention capabilities of the basement. Drainage, as most basements are below the street drainage level, requires specialist advice on dealing with any internal water from bathrooms, spa rooms and pools. Ensure the designs have prevention methods to reduce the risks of flood water entering your property. The basement ceiling and ground floor of the house is constructed at least 30 cm above the highest predicted water level, which potentially requires steps up to the house entrance.

The exterior walls of the basement will usually extend to protect any windows, entrances, and courtyards. Our waterproofing system on all the external walls creates a completely ‘watertight tub’ which prevents any penetration through the walls from either flood water or ground water. If the basement is raised out of the ground above the highest known flood level, and lightwells are not desired, it is possible to fit waterproof windows to allow natural light, while keeping water out. Engineers would analyse data to prepare structural solutions to mitigate hydrostatic uplift. This may include piling beneath the slab. Piling around the perimeter of the basement with the slab anchored to the piling. Increasing the size and thickness of the slab to add more weight to the whole structure. If the basement ceiling is finished at ground level, then we can build a one-meter-high wall on top of the basement up to the underside of the window sills, so there is a half-height wall which brings the waterproofing up to the windows. This option would require the client to install flood protection systems at all wall openings.