So are you saying the Mortgage One deal would be the way to go? 

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.

I’m just an investor. 

Right...1st I would say the tax sale wiped out any liens. 

Right click on it and select "Save as".

Quote from @Jairo Montezuma: Agreed with the above, your problem will come when you need to get someone out there on a Saturday morning around 6 am and you are three thousand miles away on vacation.

My goal was to just take his month late payments and give him a non-renewal when his lease is up.

As an investor in a market where cash flow does not come often and appreciation is a bigger play, I understand your mindset. 

Turn around and put $300k back on primary residence mortgage and use the remaining $300K to finance the next house.  

🐂 EVENT of the month….