Does that make much difference when you're choosing a plan? You bet it does! A difference of a mere 10% on a 3,000 square foot plan might unexpectedly cost you tens of thousands of dollars!
Apples To Apples
Builders, Architects, Real Estate Professionals, Bankers, Auditors, and Appraisers often all count square footage differently, to better suite their particular needs. House plan services also vary in their area-calculation protocols; in order to compare floor plan areas accurately you've got to be sure that the areas are counted the same.
Generally, builders and Real Estate Professionals want to show that a house is as big as possible; allowing them to quote a lower "cost per square foot", and making the house appear more valuable.
Appraisers and County Auditors usually just measure the perimeter of the house - a typically very rough way to calculate area - and call it a day, while Architects break the size down into components; first floor, second floor, porches, finished lower level, etc.
To arrive at an "apples-to-apples" comparison of house areas you've got to know what's included in the totals. Does the area include only heated and cooled spaces? Does it include everything "under roof" (I've seen garages figured into some plan areas!) or only "living space"?
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| Upstairs and Downstairs, Inside And Out
But even when you've discovered exactly what spaces are included in the area calculation you'll need to know how volume is counted, and whether the total is net square footage or gross.
Gross area is the total of everything within the outer edge of the perimeter of the house; net area is that same total - less the thicknesses of walls. In other words, net square footage is the part of the floor that you can walk on - gross includes the parts you can't.
The difference between the two can be as much as ten percent - depending on the type of floor plan design. A "traditional" plan (with more distinct rooms and therefore more walls) might have ten percent net-to-gross ratio, while a contemporary plan may have only six or seven percent.
Likewise, larger homes tend to have more walls - because larger homes generally have more rooms, rather than simply larger rooms.
You'll probably never see the volume of a house plan listed on a plan site, but the number representing the area of a floor plan often depends on how the volume is counted. Typically, the "upper area" of two-story rooms (foyers, family rooms) isn't counted as part of the floor plan. Likewise, stairs are only counted once. But not always - check how volume is counted to be sure you know how big the plan really is.
Plan services that design their own plans will have a consistent policy on area (and volume), but services that sell plans on consignment probably don't.
How does the designer or plan service you're buying from calculate the size of the plan? Sometimes that information is found on the service's website or book, and sometimes you have to call them to find out. But you should most definitely find out - it can make a very big difference in the cost of the house you ultimately build!
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