When I hear of historical significance, I think of an old building with significant cap ex, and functional obsolescence.

Not a major rehab - flooring, paint, and some misc. work  - not a full blown renovation.PS: I wish BP could really have a search function because the current one sucks. 

I also converted the other side of the property (it's a functioning duplex) into an Airbnb / Short Term Rental.

@Timothy Fortin As Michael's superb content here and elsewhere emphasize, cost segregation services are tightly intertwined with broader tax functions typically provided by accountants.

The city approved rental license inspector essentially just tested to make sure there was access to hot water, access to heat, no unsafe conditions like exposed electric wiring, CO/smoke detectors present/functional, fire extinguishers present/not expired, etc.

Doesn't Buildium have an accounting function though?

Once you have a shortlist, sign up for an account with each one and test them extensively to see how they function.

Would you replace a current functioning 1000 gal septic tank to a 1250 gal tank in order to list my flip as a 4 bedroom as opposed to a 3 bedroom.

It' a function of price now, and if you can find a deal that is priced accordingly, then you buy it, especially if you like the fundamentals long term

The difference in being able to apply more money to the HELOC functions exactly like making a big chunk payment to a mortgage, which significantly reduces the interest accrued over time.