I think single family homes are measure from the outside. When we had our home appraised the appraiser went around only the outside measuring. Though it doesn't get valued the same as the upper floor on the appraisal. My basement is the same footprint as the upper floor so I'm 1799 x 2. My basement is included as square footage as long as it's finished.

Multiply each room's dimensions (e.g. 15 x 12) and add the totals together. But keep in mind whatever numbers you were given when looking are likely not to be completely accurate - people round them off, fudge a little, make mistakes when figuring out 'jogs', and are just plain sloppy, plus you are right that certain areas may not be figured into the mix. The outside dimensions are just that, and those ones (on Canadian 'cut' sheets anyway, are usually given as separate measurements along with the inside ones, which are the ones you are interested in, I think.

Ditto what Lucy said. The reported total sq. footage is very often not accurate. My house was advertised as being 874 sq. ft. , but is actually under 850. Also, unfinished basements are generally not counted in my area, but that may be a regional thing. Finished basements usually are described as "including basement" around here.

Don't forget baths and closets :-)

Depends on if you want realtor square feet or usable square feet. As another poster said, when a house is appraised or listed for sale, the measurements are made outside (optimistic *smile*). So if you're comparing houses for sale, keep in mind for realty listings that although the absolute number is misleading, you should be able to compare them between listings, anyways.

Like we started with a 1650 square foot house, added 300 square feet, and when the resulting house was appraised for a re-fi, it was a 2000 square foot house. Interesting...

-Roger