Any other suggestions to improve my due diligence process would be appreciated.

Throw the junk in a dumpster, and keep or sell the things of value to recoup your costs.I never want to sell anyone's belongings.

I recently completed one on a multifamily property and found it to be a cost-effective approach.That said, there are a couple of additional factors to consider beyond the code differences between a garage conversion and new construction:Coastal Overlay Zones – If your property is in a coastal overlay, building a ground-up ADU may require Coastal Commission review, which can add time and complexity to the process.

This allowed me, for example, to purchase an automotive repair facility with an environmental issue thru BK court for $115,000, remediated to issue for $2,000, and sell the property for $325,000.  

Due to item 1, adding an ADU can require years to start achieving any return (once the accumulated cash flow recovers the initial negative position).4) Adding an ADU is a slow process.

In terms of the permitting process, an architect that can self certified drawings would reduce the wait time to receive the permit. 

Selling?

I'm selling four houses from one estate.

But what happens when:A tenant stops paying, and the eviction process drags on for months?

Once you do, then you can verify if it's hard or easy to flip there.Couple questions I would be asking, how long does the permit process take.