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 co-tenant, 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 are satisfied with your intended co-tenant's stellar qualifications, it doesn't mean the landlord will take your word for it. To increase your chances of getting an official okay, consider these questions before approaching the landlord:
Unless you are on fairly close personal 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 think about it. It is also your chance to sell your proposal by pointing out that your rental is big enough for another tenant and, assuming you already have someone lined up, that your new roommate will be a great tenant.
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 month-to-month agreement. From your landlord's point of view, this is far more than a formality, since it makes the new arrival a co-tenant who is 100% liable to pay rent and make good on any damage. 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 an additional co-tenant will probably ask for a rent increase, on the theory that more residents means more wear and tear. By signing a new lease or rental agreement, you are in effect starting a new tenancy, so the landlord can increase rent immediately, rather than give you the usual 30 days' notice (for a month-to-month rental agreement) or wait until the lease ends.
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 discriminate against you for an illegal reason -- 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 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 limit the amount of security deposits. Usually the limit is a multiple of the monthly rent, typically twice the monthly rent. Keep in mind that if the deposit is already at the maximum, but the landlord raises the rent for the new occupant, the maximum security deposit goes up, too.
For all the legal and practical information you need to deal with your landlord and other tenants, see Every Tenant's Legal Guide, by Janet Portman and Marcia Stewart (Nolo).