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Talking Point with Tony Gould

Homeowners should seek specialist help if they are considering extending their basement.

Underground extensions are the smart option for well-heeled Londoners who want to extend their homes but are hemmed-in by neighbouring properties. As a result a whole mini-industry has grown up over the past few years with some building contractors specialising entirely in domestic basement extensions.

Certainly, this is a job for specialists - projects involving excavation of basements beneath existing structures are not for general builders. It requires detailed design work and demands the expert knowledge of a qualified and experienced structural engineer as well as an expert specialist contractor.

However, the standards of safety and workmanship are often “well short of acceptable” according to a senior HSE inspector. In central London, where domestic basement extensions are especially popular, an HSE safety blitz in November 2011 resulted in one third of the 109 sites visited being served with improvement notices.

Enforcement action was taken on 40 sites, of which four were so dangerous that inspectors were forced to close the sites.

“The structural support required for a basement extension is highly complex. Not only do the sides of the excavation require support, but the buildings above and adjacent must be supported and, crucially, prevented from the effects of potential ground movement.”

Clearly any unsafe practice needs to be stamped out and I for one welcome the hard line taken by the HSE. But I suspect that the poor conditions found
on many of these sites are symptomatic of a much wider problem.

The structural support required for a basement extension is highly complex. Not only do the sides of the excavation require support, but the buildings above and adjacent - which, in London at least, often sits on shallow brick foundations set in cohesive London Clay - must be supported and, crucially, prevented from the effects of potential ground movement.

Tolerances - and hence the margin for error - are very small indeed. Envisaged lateral and vertical loads must be determined accurately and a support structure designed by experts. This is not something that should be entrusted to a general builder.

Nor is it just the building above the basement that must be safeguarded. In the sort of location where these extensions are popular, there are invariably other buildings nearby. Their foundations, too, are at risk should a neighbouring basement extend to within their zone of influence.

Before taking on a basement extension, clients and contractors must satisfy themselves that the scheme has been designed by a properly qualified structural engineer and that the correct support has been provided.

Preventing excessive ground movement that could cause damage or even collapse of structures and services as well as attending to general health and safety measures are tasks that must be carefully planned and undertaken well in advance of any construction work starting.

As the HSE inspectors found last year, health and safety hazards are relatively easy to identify. The potential for structural damage caused by poor engineering and workmanship is less evident. But where safety has been compromised, the chances are that technical standards are even lower.

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