Comps for duplexes (or other small MFR's)

[Greg Clark]

From what I can tell, comps are needed for three reasons. The first is to make sure that a duplex will cash-flow after expenses (so you would check to make sure the proposed rent price is competitive for the area), the second is to make sure the sale price is competitive for the area, and the third is satisfy any bank financing requirements. 

I make these distinctions because I've seen reports here on BP of people being denied financing because there were no available comps for duplexes in the area even though the property was being sold at a great price as far as cash flow is concerned. 

While I can't do anything about the bank's requirements, I'd like to know what sort of comps hard-money lenders look for if there are no duplexes close by. Do they just want to see that the rent expectation is reasonable (since that's what a different investor would care about and would therefore determine the selling price)? Do they want to compare a duplex (2br/1bth per side) to a 4 bedroom SFR nearby?

Thanks in advance!