If you want to see what the taxes follow the auditor's website and search the address (http://hamiltoncountyauditor.org/).  

Once you have this, visit your County Auditor Site (For me it would be Hancock County Auditor). 

I'm not sure what the new build costs per sqft are around there but maybe look on the county auditors' site and find a sfh zoned as a 2-family, and cold call it.

You can also check you local auditors website for the ability to pull this kind of public data. 

If I am not able to clearly defend myself, to an imaginary IRS auditor, I will pass on those 'expenses'.

In my opion, if you are going to be walking around town or driving for dollars there is no better solution out there then deal machine.If you want to stick to your desk and pull lists to call or mail then I'd say prop stream is probably great.You can also have a local realtor or agent you work with put you on a search for properties that have been on the market over 60 days.Another free option is that your local county auditor will likely have a way to export lists of public data on certain types of properties. 

Further, for people like me who claim Real Estate Professional, this information makes it very plain to any potential auditor that I am engaging in real estate stuff non-stop. 

I would suggest always confirming the TLA square footage on the county auditor's website. 

I have exhaustively read IRC 4975 and related docs (even the IRS guidelines for auditors examining prohibited transaction violations), and I can't find anything to support what I was told by my custodian.

Your best option is an engineering-based study unless you don't mind spending time with auditors. ;-)  Estimates are at no cost and give you the information you need to decide how it can benefit your tax situation.