The 64-year-old man from Llandovery, who has asked not to be named, was working on an accommodation block at the site when he fell three metres on 15 March this year.
He lost consciousness and suffered a bleed to his brain, a fracture to his cheek bone, bruised ribs and further damage to his existing back condition.
He now requires morphine for his back pain and has been unable to return to work.
The principal contractor overseeing the project, Evans Brothers Builders Ltd, was prosecuted by the Health and Safety Executive (HSE) following an investigation into the incident.
Carmarthen Magistrates’ Court heard that the company had arranged for the scaffolding to be removed from the site before the injured worker had finished fitting new waste pipes to the outside of the building.
He was given a scaffolding tower to use as an alternative way of completing the job, which was found to have been in a dangerous condition.
It did not have all of the protective guardrails around the platform to prevent workers falling off the edge and he had to use an unsecured ladder leaning up against the tower to climb on and off it as there was no internal ladder access.
Evans Brothers Builders Ltd pleaded guilty to safety breaches and was fined £5,000 and ordered to pay £3,685 in prosecution costs.
Speaking after the hearing, HSE Inspector Anne Marie Orrells said: “The worker sustained life-threatening injuries in the fall and could easily have been killed. His injuries will affect him for the rest of his life.
“If Evans Brothers Builders had planned the project properly then it would have known it needed to keep the scaffolding for a few more days, or provide alternative equipment so that the waste pipes could be fitted safely.
“The scaffolding tower and unsecured ladder it provided was clearly not fit for use and the moment the worker attempted to climb it, his life was put in danger.”