Arsonists are believed to have set light to a derelict wooden building.
Firefighters were called to the disused detached property in Elmstead Road, Colchester, just after 3pm on Saturday.
They fought flames between the external timber cladding and interior wall on the first floor of the property.
The operation was hampered by the unsafe state of the building and nearby power cables.
Station officer Mark James said: "There was a lot of smoke but the seat of the fire was about 15 per cent of the first floor.
"Because the building was so unstable we had to rip off the cladding from the outside using the aerial ladder."
Firefighters were able to put out the flames with two water jets without the electricity board having to shut down nearby power cables. Mr James said: "The cause of the fire is being treated as suspicious."
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