Officers found more than 250 plants in a specially built chamber beneath a back-garden outhouse, West Midlands Police revealed.
The entire operation – including lights, fans and heaters – was hidden behind false wooden panelling in the building.
In a statement, the force described the mini factory as “sophisticated”.
Inspector Stephanie Furber said: “The plants were well hidden but officers acted on their suspicions and discovered the cannabis factory.”
Officers were executing a drugs warrant at the property in the city’s Bushbury Road area on Thursday after receiving reports cannabis was being grown there.
They discovered 20 plants in the house itself before carrying out further searches on the rear building.
Two men, aged 57 and 41, have been arrested on suspicion of producing the class B drug and are being questioned in custody.
Inspector Furber added: “Drugs will not be tolerated as they not only destroy lives themselves, but can lead to further crime.
“We would urge anyone with concerns about drugs in their communities to let us know. We need the public to be our eyes and ears to help us combat this issue.”
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