It is not uncommon at all for a MLS listing to be incorrect on square footage. The problem has been discussed time and time again on this forum. Sometimes the county records are wrong; if not, it seems that it must be that the REA measures incorrectly.

That is why many listings don't have SF...as it is easier and less problematic to just list the number of rooms.

That said, as far as I know, appraisers use the ANSI method of measurement. An explanation is linked below.

Here is a link that might be useful: How does an appraiser measure the living area of a house?

Well... there is gross square footage (all the space inside the envelope... there is finished square footage--basically square footage that has drywall.. there is square footage "under heat" AKA "conditioned space"... there is net assignable square footage (inside the room walls)...

I have noticed a lot more properties being listed with an inflated square footage. Around here, some agents add up all the square footage listed in the tax records, including the porch(es). It is really misleading, and in my non-lawyer opinion, actionable.

I would measure the house using the ANSI method mentioned above. If the Realtor blatantly inflated the square footage, you may wish to discuss it with an attorney specializing in real estate law.

"Are things like bathrooms/hallways/open porches included in the final number?"

As others have said there are lots of ways sq. ft. can be taken, but I think Bathrooms and hallways are always included. I don't know why they wouldn't be, they are finished conditioned spaces. Closets also.

One common way is to take the outside dimensions of the home - excluding porches and garages - and multiply the width times the length. Of course, only postwar ranches and foursquares and colonials and saltboxes tend toward being rectangles. When a house has an irregular shape you might have to break it down to a series of rectangles, and add up the square footage of all the rectangles.

Then there is the issue of how to measure usable floor space upstairs when the ceiling slants down to meet the walls, as in a Cape Cod. That varies by location, I hear. In my old house, we counted only the floorspace that had a ceiling of at least 5 feet above it, I think. The rest of that room did not count in square footage.

Finished basements are another story. In some areas, they only count if they have daylight walk-outs, as when the house is built on a hill and the back of the basement is level with the backyard. In my area, if the basement does not have a level access to the yard, the basement does not count in square footage at all. In some parts of the country, having an escape window in the basement qualifies it as countable. In other areas, having finished walls, flooring and ceiling counts as usable square footage.

See why there is no good answer to your question? My first sentence up there could give the answer for some houses in some locations. For others, ya gotta write a book!

You measure the exterior. Garages, porches, 3 season rooms, decks, lanais, basements and attics don't count (attics count IF finished, are walk-up, heated and have windows). Basements don't count at all except as an add-on, such as "3000 sf colonial with an additional 1500 sf finished basement".

Measuring the exterior sounds easy, but it isn't unless the house is basically a box. In my case, I had to account for multiple bump-outs and a room that is really only on one floor but from the exterior appears to have a room above it. There are formulas for when there are slanted walls, such as knee-walls which Capes have. I know I found it online once when I searched for it once. The ceilings have to be at least a certain height (7'6" maybe?) and the knee wall needs to be 4', and part of the square footage is subtracted to account for the square footage from the knee wall to the minimum height requirement.

The real estate appraiser we used and the town both said the house was 4200-4300 sf. It is actually just over 4000. I am far more confident of my measurements than theirs. I am fairly certain the town double-counted the great room (which is 2 story, so no room above it even though it appears that way from outside) and the appraiser sort of rounded up the measurements (house is 32' deep at some spots and 36' at others, he just used 36').

Now, room sizes are different. Listings will often give square footage and room sizes. Room sizes are based on interior dimensions, not exterior.