Do converted 2unit houses have less market value then a comparable traditional house?

[Kaitlan Hale]

I've noticed in my area that houses that have been converted into 2 units tend to sit on the market for much longer and sell for much less than traditional single family houses with similar sqft and finishes. Is this the norm in real estate? 

Also, what about appraisals? Do appraisers see houses converted to 2 units as less valuable?