Police and firefighters were called on Wednesday night to a fire at a house in Victoria Road, St Austell.
Fire crews from St Austell, Mevagissey, Fowey, St Dennis, Lostwithiel, Bodmin and Launceston were called at 8.50pm, and an aerial ladder platform was used to fight the fire.
Three appliances and the aerial ladder platform remained on scene overnight.
Police said: "This was a house fire in what is believed to be a vacant property. No persons are believed to have been injured as a result of the fire. Occupants of a neighbouring address were evacuated and moved to temporary premises overnight.
"Fire investigators will be attending this morning along with police scenes of crime officers to try and establish the cause of the fire.
"Roads in the area have been closed and will remain closed overnight and into the morning. The roads are closed at the junctions of Woodland Road and Victoria Road and Clifden Road and Victoria Road.
"Fire investigators, along with scene of crime officers, will attend the property around 10am this morning, Thursday 21 November, to carry out an investigation. The building is deemed to be unsafe and a structural engineer is due to attend to assess the building. This may hamper a prompt investigation being carried out.
"At this time the fire is thought to be suspicious."
Devon and Cornwall Police are appealing for witnesses to the fire. If any person has any information with regards to how the fire may have started or saw anything suspicious in the area prior to the fire starting please contact the Police on 101 quoting log number 783 201113.