To determine the built-up area of a 3-storey intermediate terrace house in Singapore, consider the land area, floor area of each level, wall thickness, balconies, terraces, utility areas, staircases, and corridors. Calculate the total floor area, add wall thickness, and include additional spaces. For example, a 3-storey house with each floor at 150 sqm and 10% added for walls results in 495 sqm.