Nearly half of all basement extension projects being carried out in homes of high-value west London boroughs failed unannounced safety checks earlier this month.

The two-day blitz was carried out by the Health and Safety Executive (HSE) on homes in Hammersmith & Fulham, Kensington & Chelsea and parts of Westminster on March 11 and 12.

Inspectors visited 127 sites across the three boroughs and took enforcement action at 62 of these sites - some 48%. It represents a noticeable increase to a similar drive in the area last year, where enforcement action was required at 36% of the 107 properties inspected.

At two projects, conditions were so dangerous that inspectors were forced to close the sites.

In total, the HSE served 44 prohibition notices requiring dangerous actions to stop with immediate effect, 12 improvement notices calling on safety improvements to be made and 63 notifications of contravention identifying material breaches.

More than two-thirds of the prohibition notices served dealt with the risk of workers falling from height, either into unguarded excavations or through unprotected floor openings, as well as unshored excavations. Inadequate welfare provision accounted for half of the improvement notices.

HSE construction inspector James Hickman, whose team covers south-west London, said: “These enforcement figures reflect the rapidly-increasing number of companies entering the basement industry to meet the current high demand for basement living space in London.

“Those new to basement construction work are often unaware of the risks associated with the technically challenging nature of the work or of the standards required to ensure the safety of their workforce.

“The overall picture is similar to other targeted inspections of basement work in London where we identified the same kind of problems relating to unsafe work at height and excavations, and poor welfare facilities.

“That suggests the message isn’t getting through, or that there is complacency towards health and safety across this sector of the construction industry. But that is only partly the case.

“As well as serious safety contraventions, we also found examples of better standards, often at sites managed by companies who are known to HSE, some of whom have previously received enforcement notices requiring improvements to be made.

“It illustrates that lessons have been learned, and we hope the latest failings that required action will have a similar impact with contractors new to the basement industry.”

The inspections followed a number of fatal incidents, as well as serious injuries, in the capital in recent years relating to basement projects.

Over the last 10 years, HSE has received reports of 17 construction workers having died as a result of an excavation collapsing, while in the same period 27 were seriously injured.

Last year, a company director was found guilty of manslaughter offences and jailed after a labourer died in a basement excavation collapse in Fulham.