This article, Measurements Amiss: Size matters–on your property tax bill  struck a particular chord with me, both as an appraiser and a homeowner.
The square footage of properties found in assessor records may be well off the mark. Appraisers should use caution when pulling comps from town hall. Public records are not always accurate. After driving by the comp, if something doesn’t seem right, call the broker who sold the property (you should be doing this anyway).
Square footage is an important consideration for homeowners as well because it affects your tax bill. One of the easiest things to look at is the size of your home versus the size recorded with your town hall. Its a tangible amenity and much easier to make a case for getting your tax bill reduced. With the explosion of home additions, there is ample opportunity for errors in the tax records.
Although this pertains to a commercial building, there’s that old Manhattan joke that the “Empire State Building is more than twice as large as originally constructed.”
Concern over square footage doesn’t just apply to single family houses. Condos also see significant inconsistencies. The developer may include varying parts of the exterior space, such as a terrace, in the total square footage of the apartment, depending on the municipality. While such an amenity provides additional value, there is not an apparent standardization of how much, if any, of these sort of amenities. A small portion or all of this additional area may be included. We have seen condos that must have must have common area included in their gross living calculations. (Its a stretch, but I suspect that was intended to make the price per square foot value appear lower than competing properties.)
A below grade basement and attic space is usually excluded from square footage. However, there are exceptions. A prior home that I had owned was a Cape Cod style house that had been expanded several times that was lovingly called a “bastardized cape” in that particular market. At least 40 years prior, the attic had been converted to 2 additional bedrooms and a full bath as were most of the other capes in the neighborhood. A portion of this living area should have been considered as part of the square footage which seemed to keep our taxes low as compared to neighboring homes.
[Matrix] Length x Width Is Negotiable