Duplex have different zoning that may permit additional units.

Start with a standard offer, and write a contingency into it for electricity, all the below ground utilities, and then work your way to zoning, permitting etc.

The last you want to do is make an enemy of the firemarshall as they could permanently pull your permit to be able to rent at all.

If you did a full repipe and gutted the entire property then I assume (in all that means) that there were permits pulled and this should have been caught when the inspector did their rough in walk through or atleast the final occupancy inspection. 

I'm a contractor and just did a bid on an SB9 project in Los Angeles County - Altadena area (building permit jurisdiction).

You must get a permit, drill holes in the bottom, demo the bondbeam down a foot or so, and then fill with compaction inspections along the way..... last time I checked it was about $10,000

Some towns require a residential parking permit (sometimes on specific streets only within that same town) - you need to know what those requirements are and how feasible it will be.

It also says under the accessory uses section: "nonhabitable structure requiring a building permit." 

You’ll want to check if your specific work falls under their home improvement contractor licensing, or if there are additional local permits required depending on the county.

I'm an Architect and MRED Grad and have entitled and permitted over 200 units across Bay Area and Norther California, ask me anything about SB9 or ADU design / permitting