David Homewood, 53, from Eastbourne, was delivering a kitchen unit to Ocean Heights, Roedean Road, Brighton, when the incident happened on 3 November 2009.
Homewood was walking backwards upstairs while carrying one end of the unit while a colleague supported the other end, but as they reached the second floor landing, he stepped over a roll pack of insulation and fell down the unguarded lift shaft.
The Health and Safety Executive prosecuted principle contractor Brighton Construction Ltd for failing to manage the construction site properly.
Lewes Crown Court was told Homewood fell about six metres and suffered a fractured spine, bruised lungs and a fractured pelvis.
He was in intensive care for six days and was in hospital for four weeks. His walking is now impaired and he has been unable to return to work since the incident.
The court also heard that advice had been given to Brighton Construction Ltd after a HSE inspection five months before the incident. The inspector wrote to the company requiring that any openings in the floors were protected with guardrails.
Brighton Construction Ltd, of Maria House, Millers Road, Brighton, pleaded guilty to breaching the Construction (Design and Management) Regulations and was fined a total of £25,000 and ordered to pay costs of £15,000.
Amanda Huff, HSE Inspector, said: “As principal contractors, Brighton Construction Ltd was responsible for the safety of everyone on the site, including suppliers.
“Erecting guardrails across the lift shaft would have been easy and shows that simple precautions could have prevented this needless incident.
“Mr Homewood has suffered life-changing injuries because simple measures were not taken to ensure the lift shaft was guarded at all times.”