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You have to apply for a new Zoning Cert. Go to City Hall, 9th floor and ask the front desk person where to file for Zoning Certification. You will have to fill out the form and pay the fee. The city will send an inspector to your property to verify the two unit configuration. Things that help your case are if you have multiple gas and/or electrical meters. IF your Zoning district does not allow for a two unit building, you may have to contact a Zoning attorney and file for a Zoning change. You chould be careful about renting the unit until you figure out how to legalize it. If you have any other questions, feel free to contact me. I am an architect with a great deal of experience in this area. Good Luck!