I am a residential architect, and almost every potential client that walks in the door wants to know the answer to one question: "How much per square foot does it cost to build around here?"
For many years I had used a casual, commonly accepted number. Then about 15 years ago, after a spate of inflation, I realized I was off by about 25%, so I set out to get more scientific about tracking how much our houses cost.
As I tried to make our pricing information more accurate, it became apparent I needed to have a systematic way to compare buildings with different components in order to get a universal kind of measurement for square footage. Unfortunately, having this universal square-foot number doesn't by itself solve problems when talking to clients because they have run into many different descriptions of square feet—with no disclaimers attached.
Generally, people selling houses want them to seem big, so a 25-ft. by 40-ft. screened porch might well be counted as 1,000 sq. ft. of living space in the real-estate agent's description. On the other hand, when someone has been told that it will cost $100 per sq. ft. to build a house, they want the number of square feet to be as small as possible so that the estimated cost is less—and then they multiply only by the interior "heated" square footage and get that screened porch for free.
Of course, reality lies somewhere in between. The screened porch isn't free, but it doesn't cost as much as heated finished space. To arrive at a universal square foot that we can use to compare costs, we have developed a series of fractional multipliers that we use to get what we call "factored square feet."
We start with the gross heated square feet. This is a reasonably accepted industry standard, including the floor area measured to the outside of the rough walls. We get this information for all finished floors and total it separately because this number by itself has other uses—for solar calculations, bank appraisals, real-estate questions, etc. This number is the total gross heated space, and it is definitely what a lot of people think of as a home's square footage—but it is actually just the beginning.
Two-story spaces, we calculate and multiply by 0.5. It is an industry standard that cathedral ceilings are 1.5 times spaces with flat ceilings. (Remember, we have already gotten the basic floor space in our gross heated space, so this 0.5 is in addition to that amount.)
Does it really increase the cost of a room 50% to have a cathedral ceiling? If that ceiling has open beams and lots of skylights and balconies overlooking it, it may be more. If it is simply a cathedral ceiling, probably not—but this is an empirical way of comparing buildings, so some simplification is required. In my experience, making fine adjustments to these multipliers is not wise because it implies a level of precision that this methodology doesn't have. This is good for a first look until you get enough detail to cost it out stick by stick.