Turning a SFH into a two-family house

[Christopher Nguyen]

I used to rent out (as a tenant) a single family home in San Francisco where the upper and lower floors were separated by their own doors with locks. We shared the garage and the backyard, and the lower unit had its' own kitchen. I was wondering if you were to purchase a SFH and make this conversion, do you need to rezone (not sure if this is the right term) it into a multifamily instead of a SFH? Maybe this varies by county/city too.

If anyone has done this, are there any major complications with rehabbing the house so there are two kitchens that connect to the same gas, etc?