You must know to deal with contractors, selecting the wrong one can be a killer to your success and you must have an understanding of what to rehab for a flip versus a rental. 

Once you complete the build you can do a Cash-out-Refi and pay back most or all of the $250,000ish loan if your lenders are looking for that.If you go 5% down on a house with a basement ADU or with potential for a basement ADU & build the DADU you will have a killer community/rental property that should have very good financials.

This is almost always a financial deal killer even when zoning allows it.