The incident went to second alarm, and four fire appliances plus operational support responded to the fire.
Everyone who had been in the building was accounted for and no injuries were reported.
Fire crews had left the scene of the fire by 9pm.
The building was being renovated and Dudley said there was moderate damage caused by the fire.
"There's nothing that can't be fixed, nothing too structural," he said.
The fire was accidental, and the true cause would be determined later in the week, Dudley said.
He said the fire was a good reminder for people to have working smoke alarms in their homes.
"Working smoke alarms really are your voice at night to wake you up."
He said if anyone wanted more information or help on installing fire alarms, they should call the Whanganui Fire Station at 06 348 0103.