Unit-02 Study Notes: Estimation, Specification & Costing for Interior Design

What is Estimation & Costing?

Estimation is the process of predicting or approximating a value, quantity, or outcome based on available information, experience, and analysis. In various fields such as mathematics, statistics, engineering, economics, and project management, estimation plays a crucial role in decision-making, planning, and forecasting. It involves assessing and making educated guesses about uncertain or unknown variables.

Costing involves the detailed analysis and calculation of the actual costs incurred during the execution of a project. It includes tracking expenses, managing budgets, and determining the profitability of a project.

• Objectives of Estimation

• To provide an accurate prediction of project costs.
• To allocate resources effectively.
• To assist in budgeting and financial planning.
• To facilitate decision-making regarding project feasibility.
• To establish a basis for contract negotiation and bidding.
• To track project costs and ensure adherence to budgets.

• Types of Estimates

• Approximate Estimate: An Approximate Estimate is an initial, rough estimation of the probable cost of a construction project or any other endeavor before detailed planning and design work have been completed. It provides a preliminary assessment of the expected expenses involved in executing the project. This type of estimate is typically prepared based on limited information and is used to determine the feasibility of the project and to provide a basis for decision-making in the early stages of planning.

• Detailed Estimate: A Detailed Estimate is a comprehensive and precise calculation of the anticipated costs involved in executing a construction project or any other endeavor. Unlike an approximate estimate, which provides a rough assessment of costs in the early stages of planning, a detailed estimate involves a thorough analysis of project requirements, specifications, and quantities to produce accurate cost projections.

• Method of Preparing Approximate Estimate

• Service Unit Method: is a commonly used technique for preparing an approximate estimate in construction projects. It involves estimating costs based on the expected quantity of a particular service or item required for the project, multiplied by the unit cost of that service or item. This method is particularly useful when dealing with repetitive elements or standard components within a project.

Steps of the Service Unit Method include:

Identification of Service Units: The first step is to identify the specific service units or items that are essential for the project. These could include items such as the area of construction, the length of a road to be built, the number of units to be constructed, etc.

Determination of Unit Cost: Next, the unit cost of each service unit or item is determined. This unit cost may be based on historical data, market rates, or standard industry pricing. It should reflect the cost of materials, labor, equipment, overheads, and any other relevant expenses associated with the particular service or item.

Calculation of Total Cost: Once the service units and their corresponding unit costs are established, the total cost for each component can be calculated by multiplying the quantity of service units by their respective unit costs.

Summation of Costs: Finally, the individual costs for each component are summed up to arrive at the total approximate estimate for the project.

4.2 Plinth Area Method: is a widely used technique for estimating the approximate cost of a building construction project. It derives its name from the “plinth area,” which refers to the total built-up area of a building at the floor level, including the thickness of the walls. This method is particularly useful for quick estimations during the early stages of project planning and design.

Plinth Area Method typically includes:

Measurement of Plinth Area: The first step is to measure the total built-up area of the building at the plinth level. This includes the total area occupied by all floors of the building, including rooms, corridors, lobbies, staircases, etc. The measurements are usually taken in square meters or square feet.

Classification of Areas: The measured plinth area is then classified into different categories based on the type of space, such as residential, commercial, common areas, etc. Each category may have different rates applied to it depending on the complexity and quality of construction.

Assignment of Rates: Once the areas are classified, rates are assigned to each category based on factors such as construction material costs, labor costs, overheads, and profit margins. These rates may be obtained from historical data, current market rates, or industry standards.

Calculation of Costs: The cost for each category is calculated by multiplying the respective plinth area by its assigned rate. This yields the approximate cost for each category of space within the building.

Summation of Costs: Finally, the costs for all categories are summed up to obtain the total approximate cost of construction for the entire building.

4.3 The Cubical Content Method, also known as the Cubic Content Method or Volume Method, is a technique used for estimating the approximate cost of construction projects, particularly buildings. This method calculates the cost based on the volume or cubical content of the building. It’s commonly used when the shape and size of the building are relatively simple and can be represented as a box-like structure.

Cubical Content Methods typically include:

Measurement of Building Volume: The first step is to measure the overall volume of the building. This involves determining the length, width, and height of the building or individual components such as floors, walls, and roof.

Calculation of Cubic Content: Once the measurements are obtained, the cubic content of the building is calculated by multiplying the length, width, and height together. For example, for a rectangular building, the formula would be Volume = Length × Width × Height.

Assignment of Rates: Rates are then assigned to the cubic content of the building based on factors such as construction material costs, labor costs, overheads, and profit margins. These rates may be obtained from historical data, current market rates, or industry standards.

Calculation of Costs: The cost of construction is determined by multiplying the cubic content of the building by the assigned rate per unit volume. This yields the approximate cost of construction based on the total volume of the building.

Consideration of Specific Components: In some cases, specific components of the building may be analyzed separately to account for variations in construction methods or materials. For example, different rates may be applied to structural elements, finishes, and specialized features.

Summation of Costs: Finally, the costs for all components are summed up to obtain the total approximate cost of construction for the entire building.

4.4 The Typical Bay Method is a technique used in construction cost estimation, particularly for buildings with repetitive structural elements or bays. It involves estimating the cost of construction based on the typical or standard units of space within the building. This method is commonly used for buildings with regular layouts, such as warehouses, industrial facilities, or commercial structures with uniform floor plans.

Typical Bay Method typically includes:

Identification of Typical Bays: The first step is to identify and define the typical bays or structural units within the building. A bay refers to a repetitive segment or unit of space that is repeated throughout the building layout. These bays may include sections of floors, walls, columns, and roof structures.

Measurement of Standard Units: Once the typical bays are identified, their dimensions are measured. This involves determining the length, width, and height of each bay, as well as any other relevant dimensions or features.

Assignment of Rates: Rates are then assigned to the standard units of space based on factors such as construction material costs, labor costs, equipment costs, overheads, and profit margins. These rates may be obtained from historical data, current market rates, or industry standards.

Calculation of Costs: The cost of construction for each typical bay is determined by multiplying the assigned rates by the respective dimensions of the bay. This yields the approximate cost of construction for each unit of space within the building.

Summation of Costs: Finally, the costs for all typical bays are summed up to obtain the total approximate cost of construction for the entire building.

• Method of Preparing Detailed Estimate

5.1 The Long wall-short wall method, is a technique used in construction cost estimation, primarily for buildings with long and short walls, such as rectangular or irregularly shaped structures. This method is particularly suitable for buildings with complex layouts where walls play a significant role in defining spaces.

            Long wall-short wall method typically includes:

Identification of Long and Short Walls: The first step is to identify and classify the long walls and short walls in the building. Long walls typically run parallel to the longer side of the building, while short walls run parallel to the shorter side.

Measurement of Wall Lengths: The lengths of both long and short walls are measured. This involves determining the linear length of each wall segment along with any openings such as doors, windows, or other penetrations.

Calculation of Wall Areas: The wall areas for both long and short walls are calculated by multiplying their respective lengths by the height of the walls. This yields the total surface area of each type of wall in the building.

Estimation of Quantities: Once the wall areas are determined, quantities of materials such as bricks, blocks, or other walling materials are estimated based on standard dimensions and construction practices.

Assignment of Rates: Rates are then assigned to the quantities of materials based on factors such as material costs, labor costs, equipment costs, overheads, and profit margins. These rates may be obtained from historical data, current market rates, or industry standards.

Calculation of Costs: The cost of construction for each type of wall is determined by multiplying the assigned rates by the respective quantities of materials. This yields the approximate cost of constructing the long walls and short walls separately.

Summation of Costs: Finally, the costs for long walls and short walls are summed up along with any other structural elements or components to obtain the total approximate cost of construction for the entire building.

5.2 The Centre line Method is a widely used technique in construction cost estimation, particularly for buildings with regular floor plans. It involves estimating the cost of construction based on the centreline measurements of walls, columns, and other structural elements. This method is particularly useful for buildings with symmetrical layouts and repetitive structural components.

Centreline Methods typically include:

Measurement of Centrelines: The first step is to measure the centerlines of walls, columns, and other structural elements within the building. This involves determining the lengths and positions of these centerlines relative to each other.

Calculation of Wall Areas: Once the centerlines are measured, the areas of walls between the centerlines are calculated. This is done by multiplying the length of each wall segment by the height of the walls.

Estimation of Wall Quantities: The quantities of materials required for walls are estimated based on the calculated wall areas. This includes materials such as bricks, blocks, or concrete, as well as any additional materials for plastering or finishing.

Assignment of Rates: Rates are then assigned to the estimated quantities of materials based on factors such as construction material costs, labor costs, overheads, and profit margins. These rates may be obtained from historical data, current market rates, or industry standards.

Calculation of Wall Costs: The cost of construction for walls is determined by multiplying the assigned rates by the estimated quantities of materials. This yields the approximate cost of construction for the walls of the building.

Summation of Costs: Finally, the costs for all walls are summed up to obtain the total approximate cost of construction for the entire building.