Legal Question in Landlord & Tenant Law in Illinois

Where are you going with this? Your room mates apparently are willing to sell you out to save themselves a few bucks. Unless you have something in the nature of a written partnership agreement for as long as you rent the place, you have an oral "partnership" to share the apartment and while the amounts you currently pay are evidence that's the arrangement for this year, ... well, that doesn't mean it is binding for the renewal.

Is this a 1-bed, 2-bed or 3-bed? Larger? Do you get your own bedroom and they have to share -- point being are you really paying your "fair" share? If it is a fair arrangement it's still 2 to 1 and if this new person wants to walk into that kind of rat's nest, maybe the handwriting is on the wall. I'll also assume 3's the max in this place and not 4. Otherwise there might be a chance to everyone to win out.

As to the landlord, I'll assume the lease just says "$1,600" and does not identify the sharing arrangement you have with your room mates. Even if you and your room mates each write your own checks for the amounts you stated to the landlord to cover the monthy amount, the landlord knows you 3 share and therefore as far as the landlord is concerned how you 3 might divvy things up at any given time is your problem. And that's only for the current term, and the landlord is not going to get in the middle (especially if the other two want to stay on and there's a third to replace you). So even if you have a deal with you room mates until renewal, anything can change. If you need legal advise for this situation, what would you intend to do? Force your room mates to cave on the sharing or get the landlord to cave on something? Sounds like it's time to move on but protect yourself by making sure you get your share of the security deposit back -- asuming you contributed to it up front.

There are some potentially missing facts, such as sharing of other expenses, or whether there's anything in writing that might evidence a "partnership" but again, from a practical standpoint, the situation seems pretty clear.