The main guide lines focus on it being on a permanent foundation (Concrete slab), Hurricane tie-downs if you are in FL, GA, LA and a few other states, must be a double wide and not moved from another location.

The rental unit was probably built as an addition to our part it’s on a concrete slab, no crawl space. 

Or can you line it with concrete, and make it 30 feet deeper, with a smaller footprint, or put in several smaller ones?

There are a lot of opinions about which is better but nothing concrete.

I list in our ads that ESA authorizations must be compliant with AB468.  

I also let my pm and OTAs worry about being compliant with the new law.  

I had a consultation with them last summer and feel like I need more professional help but just can't pull the trigger for the promise of what could be without concrete guarantees it will be worth it...

One can imagine the logistical challenges of not only determining the preferred language of the tenant, but also completing a legally compliant demand notice in this language.2.

Does anyone have any advice or references to setup the structure of the business so that we are compliant and that I can qualify for conventional financing while he puts up the equity.

I can’t say that I’m familiar with the style of foundation that you have but from my experience, concrete cracks.