Anyone building a new house or putting on an addition this year needs to know the answer to this construction conundrum: When is a square foot more than a square foot? And when is it less?

The answer is important because so much in residential real estate depends on square footage:

* Appraisers and tax assessors assign market values based heavily on square footage. Their work sheets require it.

* Architects and builders project the price of buildings based on the anticipated square footage multiplied by the anticipated cost per square foot of materials, labor, overhead and profit.

If your budget is $250,000 and the architect estimates a finished cost of $150 a square foot, you will have to limit the project to 1,667 square feet. If you've got your heart set on 2,500 square feet, you must either rejigger the square footage or cut some frills.

Now back to the conundrum: A square foot is not always a square foot when you're designing a new house or planning a substantial addition. That's because different types of spaces cost less--or more--to build per square foot. If you're planning to put in a floored attic, that involves space and will count as part of the home's square footage. It will cost less per square foot than a second-floor bedroom or family room, but how much less?

Similarly, if you're going to include something dramatic on the first floor, like a two-story, cathedral-ceilinged "great room," the cost per square foot of that space will be higher than other rooms. But how much more?

A Maine-based residential architect, Robert W. Knight, has come up with a concept that attempts to answer questions like these before construction begins. Using empirical cost data from a large number of his firm's residential design projects, Knight has developed a handy rule-of-thumb system to compute what he calls "factored square footage" for a new house or addition. The system was first described in the summer 1999 issue of Fine Homebuilding, a design publication.

In an interview, Knight said his concept works for a wide variety of projects--from tight-budget, no-frills construction to higher-cost specifications--in most parts of the country. To start, you compute the square footage of all heated or air-conditioned, enclosed, finished spaces you're planning. This space, called gross square footage, is assigned a multiplier of 1.0.

Next you look at spaces that entail a cost multiplier of more than 1.0--that is, space that's more expensive to construct than standard enclosed space. A common example would be any two-story space, such as a room with a cathedral ceiling. Any two-story space is assigned a factor of 1.5 in Knight's system because, on average, he finds it is 50 percent more expensive per square foot than regular space.

Multiplier factors for other key types of square footage represent fractions lower than 1.0:

* Attics get a multiplier of 0.2, assuming the space is unfinished and unheated, with a plywood subfloor. An attic in a $100-per-square-foot home would cost out at $20 a square foot. If the attic is simply trussed space, Knight considers it as zero cost--it's just "basically part of the roof."

* Insulated garage space carries a 0.4 multiplier because it often comes with "a good deal of mechanical stuff" and has quality windows and doors. If the garage is raw and unfinished, Knight factors it at a 0.3 multiplier.

* Covered decks, screened porches or roofed decks over inhabited spaces get a 0.4 because they often require expensive finishes and detailing.

* Open wooden decks, by contrast, carry a 0.2 multiplier, meaning that a typical deck with pressure-treated framing and red cedar surface would go for about $20 a square foot in a project where enclosed space runs $100 a foot.

To use Knight's factored square footage analysis to come up with estimates for an addition or new-home project you're contemplating, create a spreadsheet.

Compute your gross "standard" square footage, floor by floor, and multiply by one or more of the cost-per-foot construction estimates typical for your area and type of project. Then do the same for the square footage of the floored attic (multiply by 0.2), any two-story space (multiply by 1.5), insulated garage (0.4), covered deck (0.4), open deck (0.2), and so on.

Knight said his factored-cost approach "isn't perfect, but it works." It "allows you to focus in advance" on some of the key economic trade-offs inherent in any project: Should we control our square footage? Or do we control what we're spending per square foot?