I just went up to some famous builders' websites and picked the most rectangular (square-ish) layouts to calculate and compare. Instead of adding one room by one room since these are almost a square or rec., I have calculated the s.f. by the length of the full unit's one edge times the other. The conclusion: it will be 50 to 70 s.f. less than what the builder indicated. The only part I have left out is the wall. So I assume all builders have some sort standard for the outer wall s.f.
Its defined by Tarion..
http://www.tarion.com/HOME/New+Home+Builders/Policies+and+Guidelines/BuilderBulletins.htm
Scroll down to builder bulletin #22
It applies to Freehold Builders/Vendors (FR), Low-Rise Condo Builders/Vendors (LR) and/or High-Rise Builders/Vendors (HR).
It defines that measurments are taken to the mid point of all neighboring walls minus an areas open to below excluding stairs... Which includes windows, hallways and neighboring units..
There is a 2% allowance for variance in size based on what they sell you in any documentation purchase and sale, advertising, sales materials..
Typically measurements are done from a middle floor unit.. IE 10th floor units n a 20 story building but this must be noted by the builder...
In reality, builders can get away with what ever layouts they want because people keep buying them.. If purchasers don't demand higher quality layouts with actual usable space they we will continue being handed crap.
Units with nice, open layouts will always seem larger and have more space due to fewer walls which further reduce the actual usable SQ of their unit.. A lot of people don't realize this because they take that 800SQ FT at face value.. You could be losing over 100 of those SQ FT due to walls, obstructions and otherwise useless areas in your not careful...