3. Detailed Estimate
A detailed estimate is a precise and accurate calculation of the quantities and costs of materials, labor, and other resources required for the execution of a construction project. It is more comprehensive than an approximate estimate and is usually prepared after the design is finalized. It involves the careful measurement of all work items and incorporates the actual rates for materials and labor.
3.1 Detailed Estimate – Data Required
The preparation of a detailed estimate involves gathering data related to various aspects of the project. This ensures that the cost is as accurate as possible.
3.1.1 Data Required for Detailed Estimate
3.1.1.1 Civil Cost
- This includes the costs associated with civil engineering works like foundations, superstructures, roof works, external works, and finishing.
- Civil costs are calculated by taking quantities from the drawings and multiplying them by the rates of materials, labor, and overheads.
3.1.1.2 GST (Goods and Services Tax)
- GST is a tax applied to the sale of goods and services in India. It needs to be included in the detailed estimate.
- The rate of GST may vary depending on the materials and services involved. For example, construction services generally fall under 18% GST, while some items like cement may have a different rate.
3.1.1.3 Contingencies
- Contingencies are allowances for unforeseen expenses during construction, which could arise due to price changes, mistakes in estimating, or unexpected work that needs to be done.
- Typically, contingencies range from 2% to 5% of the total cost of the work. It ensures that the project remains financially viable even if some unexpected costs arise.
3.1.1.4 Supervision Charges
- These are the charges for overseeing the project during its construction.
- Supervision charges are typically a percentage of the total civil costs, and they cover the salaries of engineers, site supervisors, and other staff involved in managing the construction process.
3.1.1.5 Agency Charges
- Agency charges refer to the fees paid to third-party agencies or contractors for their services, such as the hiring of labor, procurement of materials, or project management.
- These charges are generally added as a percentage of the total cost or can be fixed amounts depending on the agreement.
3.1.1.6 Procedure for Preparation of Detailed Estimate – Taking Out Quantities and Abstracting
- Taking out quantities involves carefully measuring the quantities of materials required based on the construction drawings. It includes measurements for walls, floors, roofs, foundations, and other building components.
- Abstracting refers to compiling these quantities into a summary sheet, where the quantities of each item of work are calculated and listed, forming the basis for cost estimation.
3.2 Methods of Detailed Estimate
3.2.1 Unit Quantity Method
- Definition: This method involves breaking down the work into individual units, and for each unit, the quantity of materials and labor is estimated.
- The cost is then calculated for each unit based on the quantity and rates, and the total cost is the sum of all individual unit costs.
- Example: For wall construction, the unit quantity could be one square meter of brickwork, and the quantity of bricks, mortar, and labor is estimated per square meter.
3.2.2 Total Quantity Method
- Definition: In this method, the quantities of all materials for the entire project are determined first, and then the cost of each material is calculated by multiplying the total quantity by the rate.
- This method is simpler than the unit quantity method, especially when the scope of the work is straightforward and involves repetitive elements.
- Example: The total amount of concrete needed for the foundation, walls, and slab is calculated, and then the total cost is derived by multiplying the total concrete quantity by the cost per cubic meter.
3.2.3 Long Wall and Short Wall Method / Centre Line Method
These are two common methods used for estimating walls and partitions in buildings.
Long Wall and Short Wall Method:
- This method is used when walls are constructed as part of rectangular rooms.
- It involves calculating the length of the walls (long walls and short walls), and then the areas of each wall are calculated and multiplied by the rate for plastering or finishing.
Centre Line Method:
- This method is used for buildings with walls along a centre line. It is based on measuring the centre line of the walls, which allows for the measurement of the total length of walls irrespective of the individual thicknesses of the walls.
- This method is typically used for estimating foundation and wall works in long stretches or repetitive units.
3.3 Bar Bending Schedule (BBS) for Footing, Column, Beam, Lintel, Chajja, and Slab Elements
- Bar Bending Schedule is a detailed list of reinforcement steel bars to be used in concrete works, showing the quantity, dimensions, and shape of each bar, and the total length required.
- Components of the BBS:
- Footing: BBS for footings involves detailing the reinforcement for the foundation, including stirrups, main bars, and ties.
- Column: BBS for columns specifies the reinforcement required for vertical support elements, including longitudinal bars and ties.
- Beam: BBS for beams involves detailing the main bars, distribution bars, and stirrups used in horizontal beams.
- Lintel: For lintels, the reinforcement is provided to support doors or windows, and the BBS specifies bar lengths and diameters.
- Chajja: A BBS for chajjas (overhangs) includes reinforcement for weather protection features, like projecting windows.
- Slab: The BBS for slabs provides the details of reinforcement required for floor slabs, including top and bottom reinforcement.
3.4 Provisions in Detailed Estimate
The detailed estimate includes various provisions for costs that are not directly related to construction materials but are still part of the project budget.
- Contingencies: As mentioned earlier, these are a percentage of the total cost to cover unforeseen expenses.
- Work-Charged Establishment: Refers to the labor and administrative staff employed directly for the project. Their salaries and benefits are factored into the estimate.
- Percentage Charges: These are the overheads such as agency charges, supervision charges, and other indirect costs, usually calculated as a percentage of the civil costs.
- Water Supply and Sanitary Charges: These are costs related to plumbing, water supply systems, drainage, sewage, and other sanitation work.
- Electrification Charges: The costs for electrical installations, including wiring, fittings, and fixtures, are estimated in this category.
3.5 Prime Cost, Provisional Sum, Provisional Quantities, Bill of Quantities, Spot Items or Site Items
3.5.1 Prime Cost
- Definition: A prime cost refers to the estimated cost of materials, labor, and other resources that are directly required for a project but have not been precisely determined at the time of preparing the estimate.
- Purpose: Used for items where costs are uncertain and have to be settled later. Example: Cost of materials that are yet to be procured.
3.5.2 Provisional Sum
- Definition: A provisional sum is an allowance made in the estimate for works that are anticipated but cannot be defined in detail at the time of estimating.
- Purpose: It is provided for items that are not fully designed or defined at the time of the estimate, and their scope will be finalized later.
3.5.3 Provisional Quantities
- Definition: These are estimated quantities for works or materials whose precise measurements are not yet available.
- Purpose: They allow for the inclusion of these elements in the estimate without finalizing their exact dimensions or requirements.
3.5.4 Bill of Quantities (BOQ)
- Definition: The Bill of Quantities is a detailed list of all work items with their quantities and unit rates, prepared based on the detailed design.
- Purpose: It is used to invite tenders from contractors and forms the basis for contracts, payments, and execution.
3.5.5 Spot Items or Site Items
- Definition: These are items of work or materials that are site-specific or temporary in nature. They are difficult to predict at the planning stage.
- Example: Site preparation works, temporary fencing, or mobilization costs.
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