The new I-864 form is confusing. You must read it down the first column and then up to the second column then you will see that the mailing address comes under the information about the principal immigrant, which is your relative.
Seattle Immigration Lawyer
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Your address as the petitioner.
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Use the address of the person who is signing the form.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.