Appraisers generally are looking at rents and square footage when selecting comps depending on the type of property.

If you know your square footage of walls and floors, then itemizing becomes easier and easier the more you do it.

Generally you want to find properties SOLD (not just listed) in the same neighborhood or general area with similar features, sq footage and updates that you plan to do.

May also work if the duplex is a higher price, larger square footage, in a better school district etc.I know I sound like a broken record on these forums but “Occupancy” is not always a black/White issue 

We can also evaluate on square footage of properties with similar finishes. 

Based on research of recent sales in this particular condo complex and nearby 1/1's with similar square footage I feel quite secure.

In all cases the extra footage I do not assign more value because it is not legal.In some towns it more strict than others, the violation is fairly minor.

You can do a square footage estimate - but that is only an estimate, and probably not a very accurate one until you've got a bunch of experience under your belt.You can line item all the "stuff" - this takes a very long time when you're getting started, and it's probably not very accurate either, especially when you are pricing at the HD, but a lot materials are going to come from a contractors supply (if using licensed contractors), and labor rates vary greatly.You can try to get a GC to come out and give you an idea of cost - but be aware that GC's don't like this, especially when they show up and find "another cheapskate investor trying to slap lipstick on a pig". 

Maybe down the road, that could be something to consider with this house, depending on the square footage. 

Usually houses in that area with the same Sq footage go around  $155,000 to $165,000 And that’s what I’m trying to get out of it.