The quickest way is to build a relationship with a general contractor and have them walk the property and give you a quote.

Not sure if it means anything but the property location is outside of the region that they generally do loans.... 

I believe CA market in general is expected to increase by 4-5% but SF is unique. 

It takes a certain level of QB knowledge to set it up correctly.My General Recommendation: Spreadsheets are usually fine for your first 1-3 properties. 3+ properties, you should use a software.

The area in general is booming - no better time to start than now!  

In working on construction loan financing, the lender informed me I'll need to unencumber that portion of the property (makes sense) before closing on the construction loan.I understand there to be two methods generally speaking: refinancing my mortgage and removing that portion of the property in the process OR attaining a partial release from the lender to remove that land.I *strongly* want to avoid refinancing as I have an amazing rate, but partial releases seem complex and also require a survey.

In general hands off ADU additions cost more than the value that they add.Have you looked into what the garage could rent for as a garage?  

However I have great units and we provide excellent customer service so I generally have little turnover.

Why not just target $600k buyers in general?  

Whereas with stocks you just fire and forget and generally expect a return above inflation on a bad year (obviously there's bad and good investments in both).