A growth in the popularity of basements is causing a major concern for the UK housing industry with an increase in insurance claims on basements built since 2005 costing the industry a total of nearly £21m and affecting some 890 homes, according to the NHBC.

Just 14 claims over £50,000 have cost nearly £3m over the past 12 months.

Mark Jones, NHBC’s Group Head of House-Building Standards, said: “In recent times, we have seen basements once again become a relevant and attractive addition to many houses, But as our claims figures and experience show, basement claims by their very nature are difficult to investigate and costly to repair, which can result in significant disruption to homeowners.”

NHBC figures show that large loss claims (those costing over £250,000) have risen from £400,000 in 2010 to £1.5m in 2011 and £1.7m in 2012. When combined with other smaller claims, the total costs have risen from around £3m in both 2010 and 2011 up to £4.8m in 2012. These figures are expected to rise further as some claims are still ongoing.

The NHBC has undertaken two recent surveys of around 1700 sites and highlighted key problems surrounding the construction of basements. The research discovered that tanking defects were the main cause of claims – accounting for 66% of volume and 62% of basement claims since 2006. These related mainly to the detailing of joints at ground level, floor level finishes and openings for services, pipes and cables.

It also found that 28% of sites surveyed reported high or unknown water tables, but still proposed a Type A (tanked) or Type B (Water resisting concrete) which may not be suitable for the site.

The majority of the large claims from 2012 were as a result of water ingress and incorrect installation of the damp proof membrane.