Speculation is mounting the home was used as a cannabis factory, with Sussex Police now involved with the investigation.
The force is yet to confirm how much, if any, cannabis was discovered at the scene.
A spokeswoman from West Sussex Fire and Rescue Service said she could not say whether the fire started accidentally or not.
“In terms of cause that’s not been established yet,” she said. “Nothing has been ruled out.”
Firefighters left the scene at 6.35am today, with a fire investigation officer due to return at 9am to carry out the investigation in daylight.
The fire was in Glenavon Mews, just off Clun Road, in a two-storey semi-detached house on the first floor. Firefighters were called around 4.30am.
Any drugs inside are likely to have been damaged, with the fire brigade saying the room where the fire occurred was ‘90-per-cent destroyed by fire’.
Fire engines from Worthing, Littlehampton and Arundel were all at the scene to extinguish the fire.
No one was hurt and police are yet to release any information about what was found at the house or if any arrests have been made.