The plinth area of a building refers to the total built-up area at the floor level and is an important parameter used in estimating the overall cost of construction. It is important to understand what is included and what is excluded from the plinth area calculation.

Here are the given options and their explanations:

a) Area of the walls at the floor level: This refers to the area occupied by the walls at the base or floor level. The plinth area does include this as part of the total built-up area. Walls are an integral part of the building structure and contribute to the overall floor area.

b) Internal shaft for sanitary installations up to 2 sq. m. in area: Internal shafts are vertical spaces within a building that are dedicated to accommodating plumbing and sanitary installations such as pipes, ducts, and vents. However, only shafts with an area of up to 2 sq. m. are excluded from the plinth area calculation. This means that larger shafts are included in the plinth area.

c) List and wall including landing: A list refers to a small projection or step at the base of a wall or column, while a landing is a flat area between two flights of stairs. Both the list and the landing are included in the plinth area calculation. They are part of the built-up area and contribute to the overall floor area.

d) Area of cantilevered porch: A cantilevered porch is an extended structure that is supported at only one end, while the other end is left hanging or projecting in space. The area of a cantilevered porch is not included in the plinth area calculation. This is because it is not directly connected to the main building structure and is considered as an additional architectural feature rather than part of the floor area.

In summary, the plinth area of a building does not include the area of cantilevered porches. All other options listed, including the area of walls at the floor level, internal shafts up to 2 sq. m., and lists and walls including landings, are included in the plinth area calculation.