Crews from the town and Exeter sent to the scene on Wednesday morning.
Devon and Somerset Fire and Rescue service says it received a number of calls from the public at around 5.40am.
Three Fire Appliances from Topsham and Middlemoor and an Aerial Ladder Platform from Danes Castle were sent to the premises on Fore Street in the town.
The workshop was found well alight.
A later update from the Service said: "This was a single storey commercial building measuring approx. 12m by 10m. The building sustained 20% fire damage and 50% smoke and heat damage. The fire was extinguished using 3 main jets, 6 breathing apparatus wearers, 2 thermal imaging cameras, 1 short extension ladder, small tools and lighting. The power supply was also isolated to the property."
It comes just weeks after the Fire Authority confirmed Topsham Fire Station is to close.
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