A number of crews including from Stafford, Rising Brook, Penkridge, Cannock were called to the fire inside a single-storey business on St Albans Road, Stafford, just after 2.15pm on Monday.
The flames were under control by 7pm.
Paramedics from West Midlands Ambulance Service treated three people at the scene for minor injuries and smoke inhalation. They did not need to go to hospital.
Firefighters stayed in the area to carry out an inspection of the cause. The fire was later deemed to be accidental.
Ian Read, head of prevent, protect and partnerships at Staffordshire Fire and Rescue Service, said: “We took immediate action with the fire having spread quickly and part of the building having collapsed.
“We used main jets to extinguish the fire and protect the surroundings to quickly bring the fire under control.
“The incident was brought to a close this morning with inspections having taken place throughout the night to ensure the fire was fully extinguished.”