First of all, medical marijuana growth is still technically illegal under federal law.  While state law may differ, it is still an ongoing issue for police and for the courts. That being said, you are indeed ultimately responsible for any activity and its impact conducted in your home, especially since you are not only the landlord but you live there, as well and you know of the activity.  If he grows crop in your house with your permission and knowledge, and he is arrested for violating any applicable laws (whether his intentions are lawful or otherwise), you can also be charged as a principal in the crime or as an accessory.  You cannot rent to him and not be potentially held responsible for his actions.  You may wish to think twice about renting to him.