Central County Fire Marshal Rocque Yballa said investigators believe combustible items near a floor heater on the first floor of the two-story building were the cause of the blaze.
Yballa said the fire serves as a reminder of the danger of leaving flammable objects near floor heaters, including Christmas trees and gifts as the holidays approach.
Investigators have not yet determined what objects may have kindled the fire and said investigators continue to sift through the charred remnants of the building.
The fire was reported in the 1200 block of Donnelly Avenue, about half a block from the Burlingame Caltrain station, at 3:49 a.m., according to Central County Fire Marshal Rocque Yballa.
The building housed a commercial office on the first floor and a residential unit on the second floor.
Crews arrived on the scene to find the right side of the building as well as its front porch fully engulfed in flames, Yballa said.
Nearly 70 firefighters were called to battled the four-alarm blaze, which spread into the building's attic and completely burned out the inside, according to the fire marshal.
The fire also damaged a neighboring office occupied by Tandem Entrepreneurs Management Services, located at 1214 Donnelly Ave., he said.
Crews were able to get the fire under control by 5:30 a.m., he said.
A search of the property revealed that no one was at home at the time of the fire, except for one man living in a detached unit behind the main building.
That man and possibly two other residents who weren't home at the time of the fire have been displaced.
Yballa said the fire appears to have started near a floor heater in the main room on the building's first floor.
Yballa said that the structure's age and its wood-shingled siding likely contributed to the fire's rapid spread.
The main building, which the fire marshal estimated to date back to the early 1900s, was built before automatic sprinklers were required, he said.
"If these buildings had sprinklers, it would have been contained to building of origin," he said.
A damage estimate from the blaze was not available