I'm unfamiliar with the tax laws and classifications in this regard.  

The main thing I looked up after reading the description was the zoning classification. 

Or both - SFRs that need a ton of work.Or maybe try B class if everyone else is going after C’s.

The biggest questions you have to ask yourself are: What class if the property? 

There are negative consequences for assuming this classification that you should research before you ever did this in the future.

If you're referring to property in Fayetteville, I suggest starting with  Chapter 161 of the Fayetteville Code of Ordinances which describes the zoning classifications, many of which have been recently modified. 

The JaCo formula is stated on their web page as follows:Example of Value Calculations & Tax Determination Market Value x Classification Rate = Assessed Value - Exempted Value = Taxable Value $110,000 x Residential 19% = $20,900 - $11,400 = $9,500Taxable Value / 100 = ___ x Tax Rate = Tax Bill $9,500 / 100 = $95 x $7.8368 = $744.50Tax rates are set each year by local taxing entities within the limits allowed under the constitution and state law. 

If I can ask, which service of the city would know about the classification of the fifth unit in the basement?  

Does anyone have experience with building two detached structures under a duplex classification? 

S-corp is just a tax classification when forming an LLC, you have an option to choose between C-corp or S-corp.