Martin McColl Ltd has been fined £600,000 after two people were injured after falling outside a store while an entrance ramp was being built.
A woman broke her wrist and a man broke his collarbone after falling outside the shop in Dinas Powys, Vale of Glamorgan, in January 2016.
The company pleaded guilty to breaching health and safety regulations before Cardiff Magistrates' Court.
The Health and Safety Executive said the store should have been closed.
Another firm, JMS Retail Concepts Ltd, also pleaded guilty to the same charge and it was fined £40,000.
Speaking after the sentencing, HSE inspector Gemma Pavey said: "These incidents could so easily have been avoided by simply carrying out correct control measures and safe working practices."