Stevenage Magistrates’ Court heard that Stevenson’s P&H Ltd was the main contractor at the house renovation when the accident happened on May 27 2010.
The victim is yet to make a full recovery, or return to work.
An HSE investigation found the company had failed to fit suitable edge protection.
Stevenson’s P&H Ltd, registered to Wakefield House, High Street, Pinner, pleaded guilty to safety breaches and was fined £20,000 and ordered to pay £7,373 in costs.
After the hearing, HSE Principal Inspector Norman Macritchie said: “Falls from height are all too common in the construction industry, with unsafe scaffolding often the root cause. The risks are well known and safe-working guidance is readily available, yet still entirely preventable incidents occur – as was the case here.
“The painful, potentially life-changing injuries the worker sustained could have been avoided by the simple provision of edge protection on the section of scaffolding he fell from.
“It is vital that work at height is properly planned and organised, and that all necessary precautions are taken to protect workers.”