The business decision of whether to do that however hinges on there being equity above the 1st lien and other factors that you're likely used to evaluating.

What your looking for exists but usually builders only finance on land they own, and even then its usually in a full development. 

I am looking to network with developers, contractors with commercial experience, and experienced land acquisition professionals in the Los Angeles, San Bernardino and Riverside County areas.

It's going to be closer to 1700 then 1500 if you factor in insurance/taxes... then figure prop mgmt fee (75% of first mo/renewal, 10% ea mo after) that's going to get you close to break even.

One of my favorite quotes by Jim Rohn is, "Your level of success will seldom exceed your level of personal development".If I was coaching my younger self, this is what I would recommend: Read Brandon Turner's book on Rental Property Investing https://www.amazon.com/Book-Rental-Property-Investing-Passive-ebook/dp/B018UTI2DOListening to the BP Real Estate podcast https://www.biggerpockets.com/podcastListening to the BP money podcast https://www.biggerpockets.com/moneyshowAttend your local REIA meetings to networkStart analyzing deals to get practiceEngage more on the BP forumsAll the best!

I was interested in a super-skinny strip of land adjacent to property I currently own, but there were MANY liens against it because it was owned by Brandon Barber, who'd gone to jail for misc. property development misdeeds. 

Up to you if you want to hang your hat on the de jure letter of the law or the de facto realities of the situation as it stands now.

Toll Brothers are building a development of single family homes across the street from my property starting at $500K.

The Torrance ADU development standard states "ADUs are not allowed in the Hillside Overlay or Local Coastal Overlay District unless the ADU is created from converting existing permitted square footage," which I assume are the areas you're talking about.

If I rent it out I can factor the 600 sf addition in the rent (unlike if I were to sell), but I only have 1 bathroom so I’d assume spending the money and making the house a 3 bed 2 bath would be worthwhile at the very least.