If you have more passive losses than your gain is estimated to be, you would have net taxable loss to recognize.If you want to sell and not pay tax, I would definitely look into 1031 exchange.

Given its age, we’re planning for a roof replacement and two new AC units within the next few years, which we estimate will cost around 15 - 20k in total.Our single family rental, built in 2020, is newer, so we don't expect major repairs anytime soon.On average, the duplex runs about $400–500 per month in maintenance and repairs (including significant repairs like AC replacements), whereas the SFR averages under $100 per month.

Quote from @Michael Belz: Now, I have two options: I could fix and flip it with an estimated $200K rehab for an ARV (After Repair Value) of $825K, having purchased it for $465K. 

@Vikash NandConsider books like “The Book on Estimating Rehab Costs” by J.

Here are some numbers: Current PITI: $3,970Estimated rent: $4500-$5000 (there are several homes within 4 blocks of mine that owners have turned into rentals and I've been able to compare online pics to our home, so I am confident this is a good estimate)Sq ft: 1790Estimated sale price today: 1150000-1250000Mortgage balance: 612,000I would self manage. 

If you haven't done so, you may want to get an estimate for a cost segregation study on these units.

Does BNB Calc show the lifetime of the properties revenue or is it an estimate of what it could make or is it showing the last 12 months of revenue?

The larger unit will have higher maintenance/cap ex even with coastal environments being harsh on some items.I suspect your expense estimates are far too low especially if you kept the BP calculator default percentages.  

I suspect a legal, permitted garage conversion ADU will cost ~6 times your estimate if hands off and probably at least 4 times your estimate if you do the finishes yourself.  

This is all conjecture as I am not an estimator or contractor, but I know Monteray Consan 20 works well.That was my fear with this quote, it's heavy charges to tear down and almost nothing about doing the drywall after they tear it down.