Basement extensions jobs have sprung up in the capital as home owners seek to go underground to extend properties. But a series of high profile failures have raised concerns about safety standards.

Inspectors visited 109 sites in four London boroughs, Hammersmith & Fulham, Kensington & Chelsea, Westminster and Wandsworth, over two days earlier this month.

Enforcement action had to be taken at 40 sites where 76 notices were served. Four projects were so dangerous that inspectors were forced to close the sites.

More than half of the prohibition notices served dealt with real risks of workers falling from height, either into unfenced excavations or through unprotected floor openings.

HSE Principal Inspector Andrew Beal said: “Safety standards in many basement projects are well short of acceptable, as our inspection initiative shows.

“Companies constructing basements must not be complacent about the risks. We encourage contractors, project managers and designers, to attend our free event in January and learn from those in the industry who are already working safely.”

As a result of poor standards found during the inspections, HSE has arranged a free awareness event open to all those involved in basement construction, including contractors, project managers and designers, on 19 January 2012 at Wandsworth Town Hall.