Kier has been fined £400,000 after a worker fractured his pelvis and arm in a fall.
Jair Morales was covering holes with plywood boards on the third floor of a building at Stockley Park in Uxbridge, Middlesex, when he fell 4 m to the floor below.
He fractured his pelvis and arm in the December 2015 incident and has been unable to work since.
Kier Construction of Tempsford Hall, Bedfordshire, pleaded guilty at Westminster Magistrates’ Court to breaching section 4 (1) of the Work at Height Regulations 2005.
It was fined £400,000 and ordered to pay costs of £1,534.
The Health and Safety Executive found the contractor failed to ensure work was properly planned and carried out in a safe manner.
HSE inspector Owen Rowley said the incident “could have been a lot worse”.
He said: “The system that Kier Construction has in place to control the risk from installing the protection for openings was not implemented on site, ultimately resulting in the accident.”
Kier Group safety director John Edwards said: “Operating a safe and sustainable business is Kier’s number one priority.
“We regret that on this occasion our high standards were not met and the continuous improvement of safety standards remains our focus.”