Two steel erectors, Brian Knights and Richard Bartram fell 13 m from the steelwork of the extension. Bartram landed on the decked-out floor below and managed to escape serious injury but Knights fell to the ground and was killed.
The men had been standing on a wooden staging board resting on two steel beams, attempting to retrieve a ladder, when it became unbalanced and tipped them off.
William Hare pleaded guilty to a breach of section 2(1) of the Health and Safety at Work Act 1974, for failing to ensure the safety of its employees.
HSE inspector Alec Ferguson said the accident could have been avoided if the firm had a safe system with suitable supervision.
He said: "This dreadful accident has highlighted the need for the most detailed planning and preparation for high-risk activities such as steel erection."