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Question: What’s the accepted rule for calculating price per square foot? For example, my house is a 912 square foot home, but the house is actually 26 feet by 43 feet, which works out to 1,118 square feet. But the NYC Department of Finance records indicate the 912 square foot number.

— Selling in Sunnyside

Dear Selling:

You’d think it’s pretty straightforward to figure the square footage of a property, wouldn’t you? Right out of middle school math: A x B = C.

Well, you’d be wrong.

Unfortunately, the quoted square footage of a house or an apartment can range pretty far afield. Just consider a few possibilities, some innocent, some less so:

A developer fudges a bit on the sales brochure for a new home. The “huge 625-square-foot master suite” actually measures closer to 400. A previous owner enclosed a back porch or a terrace, creating an additional 100 square feet of living space that doesn’t appear in the official building plan. You build a wine cellar in your basement so you add an additional 200 square feet to the count.

Beyond those types of discrepancies, a simple confusion over the term “square footage” can account for the difference. A raw exterior measure includes walls, columns, plumbing chases, stairwells or other uninhabitable spaces — maybe even an attached garage. In contrast, an interior measurement of “gross living area” will exclude those elements but might also miss closets or built-in cabinets.

In your case, you’ve got about 200 square feet unaccounted for. Are there any additions or alterations that can account for that much space? An enclosed porch?

How thick are your exterior walls? Eight inches would hardly be unusual, so that’s over 90 square feet. Add up the interior walls and other solid elements of the footprint, and you could have your difference right there.

Rule of thumb: You’re wisest to stick to the DOF figure. That’s the official count, and the count that you’re taxed on. It’s also the best way to compare to other properties, as you’d at least have a common reference point.

David Crook is a veteran journalist and author of The Complete Wall Street Journal Real-Estate Investing and Homeowner’s Guidebooks. Do you have a question about anything real estate-related in NYC? Write him at For verification purposes, please include your name and a phone number; neither will be published. Note: Nothing in this column should be considered professional legal advice. If you have a legal issue, consult an attorney.