The fire is mostly alluded to in the beginning. In the opening chapter, Mrs. de Winters (we never know her own name) dreams that she has returned to Manderley, the estate where she and her hudband had once lived.
However, something has gone horribly wrong. The grounds have essentially be reclaimed by nature, trees and shrubs and weeds are no longer manicured or tended at all. As the narrator's recounts her dream, she sadly observes, "The house was a sepulchere, our fear and our suffering lay there in the ruins; there will be no resurrection."
The novel is told in flashbacks. Mr. de Winter's first wife, Rebecca, had committed suicide. It is not until the end that the reader discovers the strange and probably crazy Mrs. Danvers, in her devotion to her former mistress, has likely set the fire which destroys Mandeley. Mrs. Daverswants to keep his current wife from enjoying her home or her husband, both of which she rabidly believes properly belong to Rebecca (even in death.)