Whether it's time to live with the one you love or you just need to replace a departing roommate, check with your landlord before letting a new person move in. Most landlords will insist that the new roommate become a cotenant, having the same rights and responsibilities as you do.
Obviously, you want to be sure that your new roommate is financially stable and compatible with you.
But even if you think your intended cotenant has stellar qualifications, it doesn't mean the landlord will agree. To increase your chances of getting an official okay, consider these questions before approaching the landlord:
Unless you are on fairly close terms with your landlord, it's usually a good idea to write your landlord a note about your desire to add a roommate. This gives the landlord an unpressured opportunity to consider the pros and cons of an additional tenant. It is also your chance to sell your proposal by pointing out that your rental is big enough for another occupant and, assuming you already have someone lined up, that your new roommate will be a great cotenant.
If your intended roommate passes the landlord's credit and reference checks, the landlord will probably ask both of you to sign a new lease or rental agreement. From your landlord's point of view, this is far more than a formality, as it makes the new arrival a cotenant who is 100% responsible for rent and any property damage (this is known in legalese as "joint and several liability"). It's also desirable from your perspective, because it makes it completely clear that your new roommate shares the same legal rights and responsibilities as you do.
A landlord who agrees to add a cotenant might increase the rent, on the theory that more residents means more wear and tear on the property. Under normal circumstances, landlords can't increase rent mid-tenancy—for leases, they must wait until the lease ends, and for month-to-month rental agreements, they must give whatever notice state law requires. However, by signing a new lease or rental agreement, you are in effect starting a new tenancy, so the landlord can increase rent immediately.
Unless your rental unit is covered by rent control—or if the landlord is using a big rent increase as a not-so-subtle way to illegally discriminate against you—your landlord can ask for as much extra money as the market will bear.
The landlord also has the legal right to change other conditions of your tenancy when you add a roommate and sign a new lease or rental agreement. One change that is particularly likely is an increase in the security deposit. However, this is one area where the sky is not the limit, because many states' laws limit security deposits. Usually the limit is a multiple of the monthly rent (typically two times the monthly amount). Keep in mind that if the deposit under the original tenancy was already at the state maximum, but the landlord raises the rent for the new tenancy, a new state maximum might apply.
It's possible that there will come a time when you disagree with your roommate about the rental situation. To help minimize future disputes, you might want to sit down and discuss your living arrangements and preferences, including topics such as who gets to use what spaces and who will pay the utility bills. After you've worked out the details, it's a good idea to formalize your plans in a written roommate agreement.
Also, a new cotenant means that the condition of the rental is no longer entirely within your control. Consider taking steps to protect your share of the security deposit. For example, you could take pictures or videos of the unit before the new cotenant moves in, and ask your landlord to do a new walk-through or move-in checklist. Having evidence of the state of the rental when your new roommate moves in might make it easier to settle any security deposit disputes that come up.