The crane jib appears to have sheared at around 3.30pm and crashed onto the half finished building at the Dickens Yard development in Ealing Broadway.
The fire brigade and London Ambulance crews were sent after an emergency call. They said there were no injuries.
The crane was operating on the mixed-use site which includes 698 apartment set around a square with planned shops and restaurants.
A spokesman for the Berkeley Group’s St George development arm said: “The company is investigating the matter but cannot speculate as to the cause at this stage.”