Michael McCarthy, 60, from Verwood, was contracted by PO Sharps Landscapes Ltd to construct the roof of the extension at a property in Radipole Road in the town.
The extension was being built above an existing garage which protruded out beyond the face of the main building causing a step in the scaffold at roof level of the garage.
On 20 September 2008, Mr McCarthy descended the ladder and stepped backwards over the open edge, falling three metres and fracturing his spine and ankles.
The Health and Safety Executive (HSE) prosecuting at Bournemouth magistrates' court, found the platform had no guardrails, and a ladder used to access the working platform above was positioned outward, forcing anybody using the ladder to stand backward at the unguarded edge.
PO Sharps Landscapes pleaded guilty to breaching Regulation 4(1) of the Work at Height Regulations 2005, and was fined £4,000 and ordered to pay costs of £4,000.
After the hearing HSE inspector Frank Flannery said: "Falls from height can cause significant and life-changing injuries and duty holders must take steps to prevent falls wherever possible.
"Mr McCarthy's injuries are serious and permanent: he has a steel rod in his back, steel pins in his right ankle, and left foot/heel. He suffers with chronic pain and has no prospect of working again. This incident could easily have been prevented by erecting guardrails along the platform."