Fire broke out on a property being built in Devoran last night.
The fire service said it received "numerous calls" from people about the blaze, which crews battled for almost two hours.
Firefighters from Falmouth and a third crew at Truro were first called at 7.50pm to Belmont Terrace.
As of 8.50pm the firefighters were still tackling the flames using jets.
The 15x20 metre, timber-framed single-storey property, which was under construction, was well alight.
Police were called by the fire service, as arson is suspected to be the cause of the blaze.
By 9.19pm, firefighters were damping down hotspots and breathing apparatus has been withdrawn.
A spokesman from Cornwall Fire and Rescue Service said: "Arson does not only threaten lives and damage property it has a detrimental effect on communities and the lives of people within them.
"Anyone with information on fires being deliberately started are advised to anonymously call Crimestoppers free on 0800 555 111 or contact Devon and Cornwall Police on 101 with information."
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