it seems permitted in your countyhttps://planning.saccounty.net/Pages/Accessory-Dwelling-Units.aspx

The maps generally (verification is needed) show where vacation rentals are prohibited, where they are permitted with a permit, and where they can exist and are exempted from annual permit requirements.

I am undecided on if it's best to pull permits & unsure of how I want to move forward.Would love to hear advice. 

I only say this because a tenant could be problematic, such as cause several complaints from neighbors that cause you to get fined by the city, disturbing other tenants if it's a duplex or MF, performing illegal operations, use the premises for something it's not permitted for, etc.

To be more Specific, Napa County has 41 Short term rental permits.

I would add call Bldg Dept for any open permits.3) Hire contractor or someone to do a drive by and take pictures, get a sense of condition.

But the first sentence may permit assignment of the agreement, and agents, etc., are simply wrong in saying that it can't be done.  

Each municipality also has different tests and permits that needs to be done before you can close. 

You can also decide to get the site plans approved and apply for the permits and get a construction loan.Construction loans are about 75 to 80% loan to cost so you have to put money down.

There is also a LOT more legwork on your end getting the property entitled/permitted, renderings, budgets, etc.