Can I achieve the rents or sales required to justify the development (I like 100% ROE, or 35%+ leveraged returns.

You need to determine what your Return on Equity (ROE) will be and if your money will be working hard enough for you.Regarding the duplex:The overall numbers still look dismal as a pure investment property.

@Nick Newman, $2,500 ($30k yearly) profit on $1 million equity is only 3% ROE. 

First step would be to run numbers, on an ROI/ROE basis. 

I house hacked a SFR and added an ADU to get my foot in the door a little over a year ago and the property breaks even as is and will cash flow (terrible ROE, but my goal was to get started) when I move out.

It will take a long time for the (slightly) lower rate to break even after the new set of closing costs.After accounting for vacancy, repairs, and CapEX (I assume you're self-managing), looks like you're only seeing ~2.5% Return on Equity (ROE).

And it becomes irrelevant quickly after closing when ROE becomes more relevant.Cash flow is only one component of REI returns.

Then I head the term "ROE" or better known as Return On Equity. 

Not everything is gospel but there is value.2) Learn how to repair things.3) Learn about money, COC, ROI, ROE, unrealized capital gains (I have legally never paid tax on my RE capital gains and never plan to).

However, we are expecting to have about a 20% return annually (ROE and ROA), so my busines partner and I both like the sound of 10% return on assets we just put tenants in - as well as plumbing, electrical, HVAC, insulation, drywall, paint, windows, and floors.