ARV/Comps on 2 story house

[Doug Haisten]

I am working on an offer for a house that is 2 complete stories (levels). When looking for comps icomps.com shows 2100 sq ft, Realtor.com shows 4,000 sq ft, Zillow about 2100 sq ft. The owner doesn't know for sure. Its probably the 4000 sq ft. In this town, especially this neighborhood, there just isn't many 2 story houses. and there are about 14 houses sold within the last 9 month but most all of these are average 1800 sq ft. The 3 . RE sites I mention valuate the house at between $182K,, $187K and $187K. The owner wants to use the tax assessors' value of $209K.  Would I be wrong to take 3 or 4 similar properties (1 story) and determine a $$/sq ft and apply that to the 4000 sq ft of a two story AS A STARTING POINT ?