Seven crews were called to the fire in High Street at about 2.50pm.
West Sussex Fire and Rescue Service said the blaze started in the ground floor of the property.
It is not believed any people or animals are currently inside the building.
A technical rescue unit is working to remove part of a flat roof at the back of property to give crews access to help fight the flames, a fire service spokesman said.
“We are still working to keep the fire under control and are still working to access the roof,” the spokesman added. “We are getting things under control.”
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