I don't know what the "customs" are in Chicago--some locales rely far more heavily on attorneys than others. However--and I'm not a lawyer, so this isn't legal advice--I'd definitely have it reviewed by a lawyer before I signed it. I've heard of all sorts of provisions being included in the addendum the bank sends back, and you really need to know what you're signing. And, yes, if you sign it, it's a contract. If you make changes to it (for instance, including a contingency for your lawyer's approval), then you've really made a counter offer and the bank's response (its counter offer to your original offer) is null and void.

So, unless a lawyer from Chicago is willing to commit in writing to you that it's OK to sign prior to a legal review, I'd strongly advise that you have a lawyer review it prior to signing.

I would have an attorney review it before you sign. You don't want to sign something you don't understand and have money on the line if you back out later on.