I am considering purchasing a 1940's home that we will need to add sq. footage to and vault the ceiling.  

You can use recent sales prices (don't go back further than 6 months, but paying very close attention to the last 45-90 days), calculate the dollar per square foot to use for profiles within the same square footage range, and create an AVR for the subject property based on what it will offer after the rehab is complete. 

:Would it take 48 sheets of CDX to re-sheet an entire 1,800 SF house (including the garage in square footage.

Once you have this in hand, it would not hurt to share the facts of your home with the seller of the new property such as address, square footage, some pictures, and some comps showing that you have equity to work with.

He gives me a bunch of high comps which are either in better locations,have parking,3 family homes, and 400-600 more sq footage. 

In my state, Virginia, we can set a formula, so we base that on occupancy for water and square footage for electricity. 

Individual lots completely makes sense, and the point about the exit strategy is a great one.Considering the kitchen and bathrooms make up a large portion of the cost and the remaining square footage is not quite as impactful, it does make sense in my mind to upsize the house to a more traditional size. 

Most Security systems only maintain footage for 2 weeks dueto camera memory capacity.I.

It's in a subdivision with new homes being built across the street priced in the $140's and $150's (with more square footage).  

Similar square footage, construction, quality, floor plan etc..