As per my knowledge is concerned it is depend upon builder becuase there is no such rules made by government. Loading factor is used to add constructed space not exclusively allocated to the buyer. Normally 1.25 builder is applying 25% on the carpet area. Some builders, in addition to carpet area, include terrace and balcony areas while applying the loading factor. If the project does not have lot of amenities, the loading factor should be small.In some cases loading factor of 1.3 is more than sufficient and it also includes parking space.