Three fire crews rushed to the building on Flanshaw Lane, Wakefield, at around 9.30pm on September 11 after it was engulfed by flames.
West Yorkshire Fire Service said the first floor of the disused building and roof were on fire.
Crews from Wakefield and Ossett put out the blaze.
A police spokeswoman said: “We were called at around 9.40pm to reports of a fire on Flanshaw Lane.
“We attended along with the fire service and enquiries are ongoing. It is being treated as arson.”