Carpet Area : As its name suggests, Carpet Area is the area where we can spread a carpet, means area calculated from inner wall to wall distance inside the house. This would also include steps if any, inside the house. So essentially, Carpet area is nothing but the net usable area inside the house.
Built up Area : Built up area is Carpet Area + Area of walls and ducts+ 1/2 the Area of terrace. This is usually 10% more than the carpet area. A terrace is considered as half the actual area for calculating built up area.
Super Built up Area : This is built up area + area occupied by common amenities like lifts, corridors, awnings, club house, stairs. Super built up is usually around 25% more than Built up area. This is also called as Salable Area.
Right method for carpet area calculation
Remember that loading factor is supposed to be loaded (added) and not subtracted.
Right formula is, Carpet area * 1.25 = Saleable Area
That means Carpet Area = Saleable area / 1.25 = 1500 sqft / 1.25 = 1200 sqft.
This means, you should have ideally got 1200 sqft with 25% loading.
On the contrary, if the actual carpet area is 1125 sqft (can be calculated from the layout in brochure), the super built up area will work out to be 1125 * 1.25 = 1406.25 for which you should be paying, whereas the builder charges you for 1500 sqft. i.e. for 93.75 sqft. extra.
Therefore, your yardstick to compare projects should be effective carpet area per sqft. rate and not super built up area rate.