@Nick D. I haven't personally done this, so I'm curious to hear other people's feedback. If you don't get any additional feedback, you should be able to call the City of Phoenix Planning Dept and assess the overall process, timeline, fees, and feasibility fairly easily. I just quickly looked it up myself, and it looks like they have a step by step instructions packet on their website (link below). I think the main steps include filling out and submitting an application and then attending the hearing the following month to have it voted on. As long as it gets approved, the overall process should be less than 120 days based on that packet.
It often depends on the definition of the zoning classifications within each city; based on the below info, I'm not sure which zoning classification a duplex would fall into. It appears to be a 'Dwelling, Single-Family Attached' so I'm not unclear if it changes the current zoning.
Anyways, hope this helps. Good luck!
From the COP's website:
Dwelling, Multifamily: A building or buildings attached to each other and containing two or more dwelling units. The term "multifamily dwelling" is intended to apply to dwelling types as triplex, fourplex, and apartments where any dwellings have their primary access to a common hallway or corridor.
Dwelling, Single-Family Attached: A building containing dwelling units each of which has primary ground floor access to the outside and which are attached to each other. Each unit extends from the foundation to roof and has open spaces on at least two sides. The term "attached single-family dwelling" is intended primarily for dwelling types as townhouses and duplexes.
https://www.phoenix.gov/pddsit...
https://phoenix.municipal.code...