Cost per square foot to build a house number vary widely from different part of the country to different sizes on homes, even to different levels of customization. With the latest downtrend in the market, these disparities have become even more pronounced.
Margin are now too small to allow rules of thumb to work. The margin of error is no longer able to cover these errors.
A couple of years ago, if a builder missed the cost per square foot estimate by 5% the rise in the housing value would take care of it. The owner would be willing to pay the little bit extra because the house was already worth more.
However now the case is completely different. Homeowners are not willing to cover the extra costs caused by estimation errors. This forces the builder to eat the costs, taking what has a profitable job and making it breakeven at best.
The upside of rules of thumb is they provide rapid cost estimates. Really all that is need is the size of the house and the average cost per square foot to build a house and even without a calculator builders can have a price number.
A great middle ground between the two, ultra fast yet very accurate is to use construction takeoff and estimating software.