Thought I'd give some advice about what I did for my recently sent in I-751 application. My wife (USC) has been living in a rent-controlled apartment since before we got married. We talked to the landlord about adding my name, but this meant a new lease, and re-setting the rent to current market rates. I tend to be rather frugal
So, what I did was, I wrote a letter for my landlord indicating his knowledge that I've been living at the apartment since I moved here. Our landlord is quite nice about these things, and he was happy to sign this letter.
Those of you in similar situation, or living at your spouse's parents' house etc should also consider this.
