The average cost of building a house would vary greatly depending on the country, city, town, neighbourhood it is being built in. Some of the other factors that determine the cost are time frame to build, contractors fees, material costs, furnishing costs, legal and land fees, city taxes, building regulations and not to forget the cost of housing in the neighbourhood itself. The design of the
. house itself could play a big role in estimating the cost - example a 1000 sq. ft. 1 storey ranch house will need to factor in the cost of roofing for the entire 1000 sq. ft. But if were turned into a 2 storey house the roof size is instantly reduced by 50 percent because the second floor system became the "roof" for half of the area on the first floor. This is however an oversimplified example, but there is more useful information to be found at http://www.b4ubuild.com/faq/faq_0002.shtml