The hidden risk with basement conversions

Recent research from Direct Line Insurance has found that 75% of all applications for basement conversions in the UK are for properties in the capital. While this may not be a surprise due to land values, there are risks below ground that could affect the investment.

Looking at the most recent figures, 1,968 planning applications for basement conversions were submitted to local councils in the UK in 2015. And a whopping 1,497 were in London – an 11% increase from 2013. Of these applications, 72% were successful.

The figures show that building or extending basements has become increasingly popular with homeowners wanting to increase the square footage of their properties. While loft conversions and rear ground floor extensions are commonplace across the country, the demand for limited space in the capital and land values has made building underground compelling for many.

Local authority planning submission data also shows a clear relationship in regions with increases in average house prices and the number of basement conversions.

Councils in the North East and Scotland received the least applications in 2015 at 0.4% and 0.6% respectively, where average house prices are the lowest in the UK. While London and the South East have the highest average property prices and received the most applications for basement conversions in the same period.

While the capital has the most applications, they do not necessarily pass them freely. North West councils approved 90% of basement applications; with the West Midlands just behind at 87%. All planning departments across British regions approved at least two thirds of residential basement applications in 2015.

Clearly a basement conversion requires planning permission. As the structure is affected, insurers also need to take a view on the risk, together with the addition of further bedrooms.

Groundwater – the hidden risk

What is often overlooked – especially in the capital – is the impact of groundwater driven by both the general water table, but also from hidden underground streams.

Hammersmith, Chiswick Fulham and Barnes are popular areas for basements. They are riddled with underground streams.

Ian Hogarth, an architect whose own west London basement was featured in an episode of Grand Designs, found that his house was placed directly alongside a “lost” Thames tributary called Counters Creek. Unsurprisingly, the Creek made its presence felt as builders dug deeper. It’s not surprising that at one point his builders encountered a great deal of water. While the above-ground portion of the property is a small and sympathetic extension of the mews row, two-thirds of its square footage lies beneath the house and garden.

Encountering water isn’t a reason to give up, but it does add complexity. Excavating wet clay takes a lot longer, can clog conveyor belts carrying out material and requires pumps in operation 24/7. Basements have been tanked to block the water, but they will inevitably fail over time. Today, modern systems allow water to drain down a cavity membrane inside the foundation and into a sump, where it is pumped out. Hogarth’s house has two pumps going, with a back-up for each in case of failure.

So, careful site assessment will ensure that architects and planning consultants can more accurately predict how groundwater and underground streams could affect the project cost and solutions to mitigate flood risk.

Our FloodSmart reports provide a detailed flood risk assessment (FRA) for your client’s site. They include a detailed appraisal based on the underlying geology and built on GeoSmart’s unique national groundwater flood risk map. FloodSmart meets all planning authorities’ requirements for FRAs, enhancing the pre-planning engagement process and providing clients with peace of mind.

For more information on FloodSmart or to discuss a site, call 01743 298 100 or email