The item of work not included in the plinth area estimate is the courtyard area.

Explanation:
Plinth area is the total covered area on all floors of a building, measured at the floor level of the basement or the ground floor. It includes the area covered by the walls, rooms, verandahs, and other enclosed spaces within the building. The plinth area estimate is an important calculation in civil engineering as it helps determine the overall size and cost of a building project.

Let's break down the options and understand why the courtyard area is not included in the plinth area estimate:

a) Wall thickness: The plinth area estimate includes the area covered by the walls. The thickness of the walls is accounted for in the calculation to determine the total covered area.

b) Room area: The plinth area estimate includes the area covered by the rooms. The dimensions of each room are measured and added to the total plinth area.

c) Verandah area: The plinth area estimate includes the area covered by the verandahs. Verandahs are typically covered spaces that are attached to the main building structure. Their dimensions are measured and included in the plinth area calculation.

d) Courtyard area: The courtyard area refers to the open space within the building premises. It is usually an open area without any cover or enclosure. Since it is not a covered space, the courtyard area is not included in the plinth area estimate.

In summary, the plinth area estimate includes the covered areas of a building, such as walls, rooms, and verandahs, but excludes the open courtyard area. This is because the plinth area calculation focuses on determining the covered space within a building, which is important for various purposes such as construction planning and cost estimation.