addition cost, ratio

This is just a very broad question. Let's take Case #1, a single-story home addition of 36' x 16'. Case #2 would be an addition that's 50% larger in area, at 36' x 24'.
So, would the cost of Case #2 be about 50% more than that of Case #1? Does such a rough rule of thumb exist?
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Most builders that I know base their ballpark estimates on a square footage basis. A working estimate is usaully based a cost per linear or square foot basis or per item on the items to be installed. They also factor in site specific details.
There are several factors that could kick it 10% up or down but your statement is close enough for a general discussion.
Some factors that would affect the final tally include: Crawl space or slab construction. Perpendicular or parallel to existing structure. The need for a center pier and beam on one and not the other. Added windows or doors. HVAC upgrades. And probably a couple I did not list.
If Robert Allison choses to answer you post he would give you a more accurate opinion than me.
Colbyt
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