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A second floor roof annex in Snargate Street has been destroyed by a fully developed blaze.
Fire crews from Dover, Ashford, Folkestone and Deal attended with four fire engines and a height vehicle at 7pm on Tuesday.
A Dover watch manager, who asked not to be named, said: "We contained it to its origin.
"It was a fully developed fire, the guys worked exceptionally hard to stop it spreading."
It took them three and a half hours to put out.
It is believed the building was part way through a conversion, but it is not known how the fire started.
No injuries were reported.