Can tenant use the basement with a door as bedroom?

[Rajat Bhadauria]

I have a 3 bed 2 bath unit in Cincinnati. There are 5 people living in it - 3 kids, 2 adults (Cousins). both adults are on lease. One of the cousins decided to move all my furniture in the basement and use the basement as her bedroom. The basement has a door, which opens to the backyard. It also has a Gas furnace, gas water heater, electric washer, and dryer. My question is - Is it allowed to use the basement as a bedroom? Is there any liability for the landlord?. BTW - the washer started leaking once and ruined my carpet and couch that they moved into the basement.