Police said the incident on Monday morning on the town's Howgate Street was now being treated as suspicious.
A full investigation has yet to be conducted to establish the cause of the fire.
However, police now want to trace a car which was stolen from the forecourt prior to the blaze.
The green Vauxhall Corsa - registration number Y585 JFS - was taken some time between 18:30 on Sunday and the time when the fire was discovered, at about 03:50 on Monday.
Det Con Andrew Little said: "We also want people who may have been past the garage on Sunday evening or early Monday morning to get in touch with us if they saw anything suspicious in the Howgate Street area of the town.
"This was a very large fire which has badly damaged not only the garage premises of  24/7 Automotive but also that of the adjacent property of Glass and Glazing.
"A full fire scene examination will take place when the buildings are deemed safe enough to enter."
A criminal investigation has begun into a fire at a commercial garage in Dumfries which needed more than 30 firefighters to bring it under control.