How to Work-out Sale-able area:

If  you have  purchased a flat from any builder, you would have noticed that while arriving at the sale able area , the Builder  adds certain % on the carpet area.
This % starts from 30% and can sometimes even reach up to 80% depending upon the location, goodwill  of the Builder , the type of facilities provided in the complex and of course the stature of that city.

Consider a flat whose internal dimensions are , let us say: 30’x20′.
The carpet area is therefore : 600 Sqft.

Assuming that the flat has 9″ thick walls on all 4 sides,
The external dimensions shall be:
(20’+0.75’+0.75′)=21.5′
(30’+0.75’+0.75′)=31.5

The built up area thus can be found out as :
21.5 x 31.5= 677.25 Sqft.

Thus you shall appreciate that builder will certainly desire to get paid for the built up area of 677.25 Sqft. which is 12.88% higher than carpet area.
[677.25 -600
= (77.25/600)x100
= 12.88% ]

Now assume the case of 2 neighboring flats which have got a common lobby of
20’x 5=100 Sqft. .
This lobby is shared by both the flats owners.

Since both the flats have the same carpet area , the builder shall have to load  this 100 Sqft.equally on both the flats.

Thus actual area which the builder constructed for the flat under consideration shall be
677.25+ 50= 727.25 Sqft.

Which is 21.2 % above the carpet area.

[727.25 -600 =(127.25/600)x100=21.2% ]

Summary:
Actual built-up which the Builder has to construct always is greater than the carpet area due to the walls, common spaces such as staircase , lobby ,Terrace , Parking ,Garden ,Security etc
.

For this , the builder has to calculate exactly how much extra area has been thus constructed and then distribute this pro rata on  each flat.
This method could be difficult to derive and also quite difficult to explain to the customers.

Instead of this , the builder assumes a standard % and loads this on the carpet area.

Since it is easy to calculate the carpet area from the drawing   or even by taking actual measurements at the site, builders & customers both are comfortable with this arrangement.