Crews were called to the property in Old Shoreham Road, on the corner of Grinstead Lane, at around 6.50pm
A spokesman for the fire service said the roof and the ground floor of the building were well alight on arrival.
Four pumps, three from Worthing and one from Steyning, were sent to tackle the blaze along with an Aerial Ladder Platform unit, a Land Rover Unit and Instant Command Unit, both from Hayward Heath, and three officers.
No-one was injured in the blaze which raged for more than two and a half hours.
The fire was extinguished using two high pressure hose reels, two water hydrants and four breathing apparatus. Two thermal imaging cameras were also used to check for heat hotspots.
The cause of the blaze is not yet known and an investigation is due to take place.
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