The 50-year-old builder, who has asked not to be named, had been subcontracted by Peter Steans to repair wooden window frames at a house in Hazel Grove, Hallaton, near Market Harborough in Leicestershire.
He was working from a ladder extended to just under the window when he dropped a piece of window bead. As he started to climb down the ladder to retrieve it, he fell to the patio below, smashing his workbench on the way.
He was airlifted to hospital with head injuries, five fractures to his spine and a fractured pelvis and wrist.
The Health & Safety Executive (HSE) investigated the incident and prosecuted Peter Steans for safety failings at Leicester Magistrates' Court.
Magistrates were told that the incident happened on 14 September 2012 when the worker was cutting out and replacing the sill and glazing on a small window in the second floor attic gable wall.
HSE's investigation found a suitable tower scaffold was on site that could have been used to remove the risk of working from a ladder.
Peter William Steans, 61, of Lubbesthorpe Road, Leicester, pleaded guilty to breaching Regulation 4(1)(c) of the Work at Height Regulations 2005. He was fined £4,000 and ordered to pay costs of £353.
HSE inspector Tony Mitchell said after the hearing: "The injured man had to use a walking frame and was unable to climb stairs for five months. He will never be able to return to his normal job, yet the incident and the life-changing injuries it caused could have been prevented.
"There was suitable equipment on site and Mr Steans, as the contractor in control of the work, had a duty to make sure it was put in place for use. Builders should not think that just because they use casual workers, they are not responsible for their safety. If you are in control of the work then you will be held accountable should things go wrong."