Other thoughts to consider -  it might be good to add a dedicated water heater and heating/cooling system to the ADU - so you aren't fighting with the tenant over the temperature or who is using too much hot water.   

This is the hottest area of my city, new residential buildings are going up, renovating old warehouses and industrial parks with urban apartments.

About 25 years ago I moved exclusively to commercial - basically free standing retail, warehouse, auto repair, etc.

Plenty of strip malls and warehouses in my town could use some help and advertise as build to suit.

.+) mixed use building (warehouse, church, school)  that I can be creative with and do a larger Airbnb concept.

Floor and Decor, Builders Warehouse, and Home Depot and Lowes of course.

This is because 1-4 unit buildings can qualify for residential loans, 5+ unit buildings are considered commercial.Forced appreciation only applies in commercial properties - multifamily, storage, mobile home parks, warehouses, office buildings etc.

I would look in the south county Riverview, Gibsonton, Ruskin area. lots of growth down there. plus they are talking about another amazon warehouse down that way.

Maybe bigger multis, I think warehouses in the design district would be fun!

It's my first time buying an industrial warehouse in Southern California.