Firefighters including Martin McKilligin, watch manager at Storrington Fire Station, entered the home in Hog Lane after neighbours raised concern for the woman.
Mr McKilligin said the fire is believed to have started accidentally.
He said: “Neighbours suggested the elderly lady was in the property because she is housebound.
“It was through the neighbours and information from them that gave us where the lady is normally sitting during the day.”
Appliances from Storrington, Chichester and Littlehampton initially attended the scene, and were later joined by teams from Arundel and Worthing.
Mr McKilligin added: “We attempted to apply breathing apparatus but the property was so thick with smoke.
“It was very hot and very smokey - every room had blackened windows which would suggest the fire had been contained in some considerable time.
“Unfortunately the lady’s body was found on the ground floor.
“This is just a sad accident.”
He added that police are continuing to investigate at the property.
Fire crews were alerted by an automatic alarm and 999 calls from neighbours at around 4.25pm.
Inspector Steve Rayland of Sussex Police said: “It has been confirmed that there has been a fatality found in the house, but next of kin have yet to be informed.
“The fire is now out but part of Hog Lane remains closed.
“Police and fire investigators are at the scene and working to establish the circumstances of the fire.”
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