I typically advice a cash out refi over a HELOC if you have an immediate use for the funds since HELOCs have variable interest rates, but you also want to take your current interest rate into consideration.

You can run your own comps on Zillow/redin  make sure you understand your local market and how much finishes, bed bath count effect prices, don’t lie to yourself about arv or project it to go up that’s how you end up in trouble.if you can get all those variables down, while things can still happen/go wrong you should have a good ball park of the potential profit margin, always room for error in your net profit, markets can shift, you can get hit with unexpected expenses etc. 

Rate would be around 8.15%, but it's variable.* [Soon] Buy a condo with that cash, live on it, and pay the loan interest (as if it was my rent), basically making this an interest-only loan, where I can pay the premium whenever I want, deducting the "rent" as I go. * [6-24 months] Whenever I find a job and qualify for a FHA multifamily, get that property (with break-even cashflow most likely), and rent the condo at a cashflow loss for a couple years until rent increases (due to high-interest on underlying debt)* [Almost immediately afterwards] At this point, do a cash-out refinance on the condo to pay back the SBLOC (unsure if this is possible immediately after getting the multifamily loan), and secure a lower rate for the condo itself.I know this strategy sounds like loser, as it bleeds money all around, but my hypothesis is:* Underlying assets should grow more than 8.15% in a given year, and I would not pay short/long-term taxes on stock.

Often ARMs have a threshold for how much the rate can go up or down once the variable period starts. 

@Russell Payne there are too many variables to give a flat answer.

I hope the example illustrates why HELOC to finance ADU is not optimal not even addressing the risk of a variable longer term loan (I do not use variable on longer loans) or additional interest rate (these are secondary issues but I would have issues with HELOC even if the HELOC were fixed at the same rate as a new RE purchase loan (but HELOC are typically variable with worse terms than RE purchase loan). 

Determining whether to do a cost segregation study or not has a lot variables.

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However, you may find that they offer loans with shorter terms like 15-20 years and possibly with variable rates instead of a fixed rate.

Your investing career will be determined by your ability to accurately predict variable expenses, vacancy, maintenance, and CapEx.