Police who raided a Bedford house found 400 mature cannabis plants in one room.
A man in his 30s was arrested and growing equipment was seized at the cannabis factory in Ampthill Road.
Sgt Debbie James, of Bedfordshire police, said: "Any property could be converted into an illegal cannabis factory even though from the outside the house looks normal."
She asked residents to look out for signs of cannabis factories and to pass suspicions to the police.
"If anyone has suspicions about what their neighbours are up to or who they are renting to they should contact us," she said.
"In this case the 'gardeners' made the house appear as normal as possible to minimise suspicion."
Classic signs of cannabis growing include permanently drawn curtains or blinds, a strange, strong smell and people arriving and leaving regularly throughout the day and night.