Emergency services responded to the Heath Road residence around 11.20am after a witness sighted smoke coming from the property.

"CFA and police investigators have deemed the fire not suspicious," Portland Detective Sergeant Jason von Tunk said.

"The fire started in the lounge room of the house, and was possibly caused by the open fireplace.

"Items inside the house have caught fire, the house is going to have to be totally destroyed."

Detective Sergeant von Tunk said the homeowner, a 45-year-old Portland man, may have left the door of the fireplace open before going out for the day.

He said the home was cluttered with items, some close to the fireplace.

Portland police Acting Sergeant Leah Sullivan said the house was now "uninhabitable".

"The house was badly damaged, making it uninhabitable," she said.

"Police are working in conjunction with the CFA fire investigator."

"All occupants have been accounted for."

CFA commander at Portland Terry Heafield said it took almost an hour to bring the fire under control.

"We had firefighters in breathing apparatus to do an internal attack and search if anyone was home, which there wasn't," he said.

"The fire was intense, it caused significant damage.

"This is a timely reminder about home fire safety. Even though it's starting to warm up, people must remember not to be complacent with fireplaces, keep items clear of fireplaces and to ensure they have working smoke alarms."

EARLIER: Police are investigating 'suspicious circumstances' surrounding a fire that fully engulfed a Cashmore home on Monday morning.

Emergency crews were called to the house fire on Heath Road at 11.20am.

They found the house fully alight, a CFA spokeswoman said.

"On arrival they found the single-storey home fully involved," she said.

"They don't believe anyone was in the premises. They have done a search and haven't found anyone yet.

"It's still going, they haven't brought it under control yet."

Police were called to attend for suspicious circumstances, the spokeswoman said.

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