I am wondering how developer calculate build up for a property? Do they take into account porch area, belcony, etc? Based on the layout on developer's brochure, I calculated less sq feet than advertise. This doesnt include porch area and some portion of the balcony.

I've been told it is everything under the roof including porch, balcony, patio etc

I think this is true. better to underestimate than overestimate

it depends. usually advertisement puts up the land area rather than the house itself (eg. 20x70 etc).

even when it's printed in the brochure, it could end up being lesser than that which is why if you're purchasing directly from Developer, there's a clause in your sale and purchase agreement which allows you to claim if the difference is 3%? (need to double confirm on the difference).

Thanks for the answers.

The thing is the brochure and the size in the SNP are different. The SNP size is much smaller.