In WA there have been a number of cases where developers have promoted their projects based on architectural area. This in fact measures from the outside of the external walls and centre of the party walls. However when the buyer signs the final contract the size of the apartment will be shown as strata area.
Strata area is measured to the inside of external walls and party walls.
Often there can be up to 10% variation in these two sizes and so it is very important when shopping around that buyers ask what measurement is being used in the promotional material.
In the East Coast there are standards set as to the use of strata area but as of yet these have not been introduced into WA.
Buyer beware - but more importantly make sure you ask for clarification!