BASF Construction Chemicals
BASF Construction Chemicals

According to research released by Direct Line Insurance in 2017, applications to build or extend basements in residential properties increased by 9% between 2013 and 2015, with a massive 75% of all basement planning applications in 2015 being made in London, where house prices are highest.

As basements become a popular option in the capital, there are knock-on effects: not only do newbuild basements require planning permission (cellar conversions may not), but insurers must also assess the risk to the structure affected, as well as the addition of further bedrooms. In some of the most popular areas of London for basement construction, there’s the added complication of groundwater, not only from the from the water table but hidden underground streams.

Between 2005 and 2013, claims related to waterproofing below ground cost NHBC in the region of £21 million and affected around 890 homes. The NHBC says: “The number of claims relating to basements and tanking failures is relatively small. However, we have recent experience of individual claims costing up to £500k. These high costs, and the disruption that can result when they occur, highlight the importance of ensuring basements are designed and built correctly.”

You can download NHBC Standards Chapter 5.4 ‘Waterproofing of basements and other below ground structures’ here, and there’s more information from NHBC here.

What does insurance cover?

According to the Financial Services Ombudsman: “Insurance policies for properties – both residential and commercial – usually provide cover for any loss or damage that is caused by flooding.

“Most policies do not define what a ‘flood’ is. So we often see cases that involve a dispute over whether a flood was actually the root cause of damage. We also see complaints about the way claims have been handled.

“We sometimes see cases that seem to be about flooding, but that are really about the quality of the repairs that were carried out after the damage was done – or the time it took to complete those repairs.”

Who is liable?

The Financial Services Ombudsman says: “We sometimes see cases where a basement room – that was watertight before – has started to let water in, and the insurer says that the damage was caused by failed tanking – and that the tanking had failed because of wear and tear and/or lack of maintenance. Generally, we say that the cause of the damage was flood – not the failure of tanking. But if there is evidence that the consumer should have known that tanking was defective or missing before the flood happened, we may say it was reasonable for the insurer to reject the claim.”

20 Inspiring Basement Designs

Play it safe at design stage

As waterproofing consultant Ian MacLennan points out, you must have a waterproofing expert on the design team, and the designer of the basement waterproofing must have professional indemnity insurance (PI) specifically to cover the design of waterproofing. In addition, products insurance to cover all eventualities in the event of a problem with the waterproofing is essential. Without products insurance, guarantees are of little value as most won’t be covered by insurance.

As a sense check, always ensure that the design process is fully compliant with the requirements of BS8102. That is:

  • The waterproofing system has been designed by a specialist
  • A geotechnical specialist has conducted a site investigation
  • Any faults in the workmanship/materials are designed out
  • Repairability is considered and the feasibility of remedial measures assessed


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