Note: The method of obtaining quantities of items involved is called ‘quantity estimate’, and the method of obtaining unit cost of items involved is called ‘Rate analysis’.
The various types of estimates associated with a construction projects are as follows:
1. Preliminary or approximate estimate:
The preliminary estimate should be accurate enough to keep the cost of project within 10–15% variations.
2. Rough cost estimate/plinth area estimate: Plinth area of a building = Length × Breadth, excluding plinth offsets.
Generally, rates are arrived by dividing total cost of construction with its plinth area. Using this rate as the basis for next construction, approximate cost of the project can be obtained.
The documents attached with this estimate are:
Cubic contents estimate: Cubic contents of a building = Plinth area × Height of the building.
The height taken here is from top of the floor level to top of the roof. The procedure for cost estimation and accompanying documents are similar to plinth area estimate.
3. Detailed estimate:
4. Revised estimate: Generally, a fresh and revised estimate is prepared when the sanctioned estimate exceeds by 5%. A comparative statement along with the reason of increase of cost should be attached to the last page of the estimate.
5. Supplementary estimate: Detailed estimate in addition to the original sanctioned estimate is prepared when additional works are deemed necessary during the progress of work to supplement the original works.
Generally, the type of estimate carried out depends on the purpose and the level of accuracy needed.
For example, estimate for the material supply has to be very accurate, whereas to estimate the cost of the feasibility studies of a project, the accuracy need not be very high.
Estimation can be done by applying various methods. A method chosen may depend upon the design and shape of the building.
The various methods of estimation are given below.
1. Centre Line Method
2. Outtoout and intoin Method
This method is widely applied under all circumstances.
3. Bay Method
4. Crossing Method
Lengths and breadths of the masonry walls at plinth level are taken in this method. Symmetry is essential to do estimate using this method.
5. Service Unit Method
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