"It took them just over three hours to extinguish the fire - it was in two floors and the roof."
He said the fire was "definitely suspicious".
Police are understood to have been alerted to burglaries in the area about the same time, but Nelson police spokeswoman Barbara Dunn dismissed those reports.
"That suggestion might have come from the sounds of glass popping and smashing, so at this stage, no suggestion that there's been a burglary at the scene of the fire."
Ms Dunn said the cause of the blaze was yet to be established, but it was considered suspicious.
Fire safety officers and police would investigate this morning.