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 already have some equity in the property after doing a complete rehab but I don't want to diminish my returns selling it after one year, paying for commissions/closing costs.

Modular, on the other hand, which I also deal with, you can do a mod and put it in the city limits but would cost as much or more as a stick built, more if you go with a two story.You're much better off going with an existing duplex

A lot of BP Pro Members are landlords so they need a software to manage those properties, so the RentRedi Perk saves members around $360 in software costs. 

cost of doing the deal. 

With the lack of housing and the cost of everything going up, more moderate practical housing spaces might be a thought. 

Because we have a very large community of investors that buy thousands of homes across the country every year, we are able to negotiate better buying opportunities (and loan options) than you would likely find on your own as an individual investor.

Background on me: I need to obtain a graduate degree for promotion at work, but it can be in any field at low-no cost to myself.

Assuming the loan would likely be best from a cost/fees (and likely interest rate perspective). 

It's important to run your own comps and go out to the property with a contractor so that you can get an idea of true renovation costs.