Leasing Individual Rooms

[Tom Squire]

Hey BP! I am currently in the process of buying a duplex to renovate (cosmetically) and live in one side.  Right now my GF and I are sharing a rental house with her cousin and his GF. We are planning to offer them to rent the other half of the duplex, but we know they wouldn't be able to afford it on their own. We're willing to let them rent out the other room in the house to a friend or someone from craigslist to offset the cost.  

If we go down this path, what is the best way to structure the lease arrangement for the that half of the duplex?  Should we have 2 leases, one for each room and have the shared common space?

What else should I be considering with this arrangement?