After completing renovations, the estimated appraised value has increased to $370,000.

Is $800k a good estimate (~ $73k door)?

The second option at least gives you a chance at a fresh start with a new tenant.Of course, a new tenant is an unknown and there's no guarantee they will be better.

The loan was for a 12-month period as the build was estimated to be around 10 months.

Refi after construction.These numbers consider only the portion of costs of the HEL attributable to the land purchase, not the payoff of the HELOC (which we took out to buy the Seaside condo).Cash In:                     $66,166 (Cash, 1 year of debt service of HEL, debt service of const. loan, furnishing)Amount Financed:      $548,000 (home equity loan + construction loan + closing costs)Total Cost of build:     $614,166ARV:                               $850,000 (or rather "after construction value")Refi                                $637,500 (75% of value + closing costs)                                        Cash Out                       $89,500New payment             $4500/month (54,000/year)Estimated Cash Flow (pre-tax numbers, so actual mileage may vary)airBNB year 1:            $70,000     (net income $16,000)airBNB year 2:            $100,000   (net income $46,000)airBNB year3+:           $120,000   (net income $66,000)ROI (construction year):    0ROI Year 1 of STR:              24.2% ROI Year 2 of STR:              69.5% ROI Year 3+ of STR:            99.7% Did I calculate these ROI numbers right? 

Chances are they are "more knowledgeable" that others in that niche.

First of all, let me pay my respects to the hardworking people in real estate and the treacherous waters they navigate—fronting hours of effort with only the chance of commissions and an abundance of non-forthright people out there.

Its current projections are to bring in between 150k and 180k with estimated costs of 60k.  

You will have to have a professional estimate done.

Airdna will give you a good estimate of the demand for the area.