Police found cannabis plants with a value of £370,000 when they raided a derelict property in Hartlepool.
Officers raided the old Manor House in Owton Manor Lane on Thursday evening and found 445 plants of the drug.
An investigation has been launched into the discovery, but so far no arrests have been made.
A Cleveland Police spokesperson said: “Our officers are dedicated to ridding Cleveland’s streets of drugs to improve life for those in our communities who are affected by those involved in drug activity.
“We would continue to urge the public to come forward with information on drugs activity in their local area by calling Cleveland Police on 101 or Crimestoppers anonymously on 0800 555 111.”