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How to hold basement builds to high standards

Basement projects remain a cause of concern in the housebuilding industry.

With the current inflation in house prices and limited availability of land, it’s no surprise more developers are extending below ground.

It is not uncommon in large houses to extend three storeys deep below ground, creating the new era of ‘iceberg’ houses, potentially doubling or even trebling floor areas to maximise returns.

This boom of basement construction has uncovered several key issues, mainly: poor workmanship; health and safety; and planning issues.

Inspection blitz

The HSE has experienced an alarming manifestation of basement construction health and safety issues.

As a result of the concerns, the HSE has initiated several inspection blitzes on basement sites across four London boroughs over the past two years.

The inspections have resulted in a third of basement sites failing health and safety spot checks, resulting in enforcement action. 

Despite the clear evidence that domestic basement projects remain a cause for concern, HSE’s lead inspector for the initiative believes some progress is being made.

“In the Royal Borough of Kensington and Chelsea, planning applications for basements rose from just 13 in 2001 to 182 in 2010 and 307 in 2012”

The local authorities, especially in our major cities, have also experienced an influx of basement planning applications – and subsequent issues.

In Kensington and Chelsea, planning applications for basements rose from just 13 in 2001 to 182 in 2010 and 307 in 2012.

In March 2014, the London Assembly unanimously agreed a motion to take action to limit excessive subterranean developments across London.

Kensington and Chelsea appear to be leading the way, with significant changes proposed to their planning rules, limiting domestic basements to single storey below ground level and a maximum of 50 per cent of any rear garden.

NHBC campaign

In 2013. the NHBC launched a basement campaign that highlighted a number of workmanship issues.

Between 2005 and 2013, basement claims cost NHBC in the region of £21m, affecting around 890 homes.

With the frequency of basement claims to registrations occurring 1,600 times more than foundation-related claims, there is a significant challenge to ensure the quality of build.

NHBC has been working with key figures and organisations from the sector on various initiatives to address the problems; one of these initiatives includes implementing a new basement chapter within the NHBC Standards to be delivered in 2015.

This new chapter will provide practical guidance for structural waterproofing and the construction of basements.

What can we do?

Given the past failure rates, the current boom and the impending reform, the basement industry has a significant challenge ahead. 

So as designers, what can we do to help reduce risk and raise standards? Key areas are discussed below:

  • Be satisfied that the chosen waterproofing system is suitable for the performance grade in question and level of risks involved.
  • Ensure the extent of additional investigations specifically required for the assessment of ground conditions, water levels and design of the basement construction reflects the level of associated risk involved. 
  • Check that all critical construction points within the chosen waterproofing system have been suitably detailed prior to construction.
  • Satisfy yourself that the contractor is competent to install the system in question.
  • When value engineering, ensure the waterproofing system is not compromised and still reflects the level of associated risk involved. 
  • Only use products/systems that have current independent certification or are of equal or better level of assessment.
  • Keep up to date with basement guidance and legislation – NHBC CH5.4 (Due 2015), new planning requirements (due imminently) and The Basement Information Centre New Guidance Document to replace AD Basements for dwellings.


Marc Separovic is technical projects manager at NHBC. Email technical@nhbc.co.uk if you have any queries.

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