When your bank performs their mortgage appraisal they will measure to verify the listed square footage is accurate. It's very uncommon for properties to be off by a few square feet, so don't panic.
Prudential Castle Point Realty
It can depend on the architect sometimes. Not all architects and developers measure square footage the same. Some count from the wall in - closets included. Some count from a quarter of an inch or more outside of the sheet rock then inside the walls.
For the most part square footage is measured from the walls in and closets are included before doors and all walls are put up. Square footage can change because of last minute adjustments and floor plan changes and the deed and condo docs are not always the same.
I am not sure what you mean by open space, I think you mean space that you can actually walk through. In that case, then yes because again, closets are included in that number.
So a 1080 sq foot home could have only say 900 square feet of livable space.
Outdoor space such as terraces and patio's are never counted in square footage - or I should say, SHOULD never be counted in square footage. Although, once in awhile I will see a listing that does count the outdoor space, but that is incorrect. Outdoor space is measured separately.
Also, just want to note, that when an appraiser comes in to appraise the home, he or she will also measure the space to count square footage and draft a floorplan. Most of the time, their number does not match up with what the docs say.
Hope this was helpful!
Don't hesitate to reach me if you have any other questions.