First, make sure you understand that carpet area and built-up area are two different things. An easy way to remember this is, the carpet area is all the space indoors where you can lay the carpet. This includes rooms, bathrooms, etc. Here, you don't calculate the thickness of the wall either.  But when we talk about the built-up area, it includes other parts of the home. This includes balconies, terraces, etc. We have a comprehensive chart to help you understand and see which part of the home falls under which category. You can look at the above article for more details.