Hello,
In contracts it is usually specified what the total built area is and what the "useful" area consist on (the area without proportional part of commonelements of the building). This will also be the way of describing the property in the deed.
In some cases, even a description of the diferent areas of the different elements of the property will be specified, but it is usually included in a plan of the property as an attachment of the contract. This, together with the memory of building specifications and the book of the property (indicating the electric and water installation, materials, diagrams of other installattions, etc.) have to be given to the purchaser by Law (usually the building specs. together with the contract, in order to know the qualities of the property, and the final book on completion deed).
Unfortunately, not always the developers specify the total built area and sometimes the contract uses an expresion referring to the property in general, whatever the final area is, or even establishes that a possible difference in the areas of +/- 5% could be allowed.
If you have doubts about the apartment or house areas always ask your lawyer about the contract or Registry information. If the property has already been registered in the Land Registry that is the legal area that you will be buying.
I hope this clarifies a bit.