Carterton fire chief Wayne Robinson said it was a relief no one was home.
"Normally we turn up at a house at night, someone's there," he said.
"We had three crews, we searched the house, and no one was in."
Ironically, the fire was close to Mr Robinson's house, meaning he had to drive by it on his way to the station to get kitted up.
He said it was "going well" when they arrived, but with 30 firefighters it did not take long to bring under control.
"In an old building, all timber, and sarking covered with gib, there's these different layers, which keep a lot of hot spots in."
He said police and a fire investigation officer would attend the site to determine the cause of the blaze.
"It's being treated as suspicious."