An electrical fault on equipment being used to cultivate the illegal drug started the blaze at a house in Westhoughton in Bolton on Thursday.
Police discovered the remains of the drugs farm in Townsfield Road after they were called out by fire crews.
Det Ch Insp George Fawcett said the fire was a "clear example" of the dangers of cannabis farms.
'Potentially lethal'
At least three rooms at the house were being used to store the plants.
"The electrical equipment used to run these farms is potentially lethal, which is why it is vital that the public pass on any information they have about houses being used in this way," Det Ch Insp Fawcett said.
"It is difficult to say how many plants were inside the house as it has been extensively damaged, but it is clear that at least three rooms were being used to store the plants."
Greater Manchester Fire and Rescue Service said the fire caused severe damage to the ground and first floor of the house, and damaged the roof of the house next door.
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