The 37-year-old man was working on a house in Ellerby Street, Fulham, on Thursday afternoon when the floor fell through.
He was caught under the rubble and died at the scene.
A Metropolitan Police spokesman said the Health and Safety Executive were involved in the investigation.
“We were called at 1500 GMT to a building collapse,” he said.
“A man was pronounced dead at the scene.”
The man has not been named. A post-mortem examination will be held today.
The work was being carried out by Hertford company Siday Construction. The firm did not responded to the BBC’s request for a comment.