The 31-year-old male, who does not want to be named, was visiting the home of a friend in Port Isaac, when the fall occurred on October 6 last year.
The large excavation was in a neighbouring garden and had been dug out as part of a major domestic refurbishment project.
Bodmin Magistrates’ Court heard that HML Builders Ltd, of Wadebridge, was responsible for the work.
An investigation found the company had not installed any edge protection to stop people falling into the pit, despite the edge of the excavation running along a path directly to the front of a chalet on the neighbouring property.
HML Builders Ltd pleaded guilty to safety breaches and was fined £12,000 and ordered to pay £3,793 in costs.
After the hearing, HSE Inspector Helena Tinton, said: “The failure to put any edge protection in place to stop people falling into the excavation could easily have caused a death.
“It was dark at the time of the fall and the injured man had no reason to know the hole was there.
“His serious injuries could have been avoided by some very simple measures, such as suitably supported guard rails and toe boards.
“HML’s negligence not only put the lives of their own workers at risk but also those of members of the public.”