Employees at builders merchant Travis Perkins were evacuated when a fire broke out at the branch in Guildford.
The fire broke out near the kitchen area of the building on Thursday afternoon and quickly spread to damage the roof area.
Eight fire engines, police cars and an ambulance attended; the fire was eventually put out by eight hose reels and crew using a hydraulic lift.
Passer-by Ade Goldsmith told Get Surrey: ” The whole building was alight – it was the biggest fire I’ve ever seen. As I got there the fire engines were just turning up.”