This article will focus on Basic Types of Estimates used in quantity surveying.
What is an Estimate?
An Estimate is what determines the probable cost of a project, which can be a building, a car or it even can be small lightning bulb.
Types Of Estimates In Use
Types of Estimates can be divided based on how project is quantified. Also the level of accuracy required will dictate which estimate is suitable for a particular project.
Three broad types of estimates are listed below
- Detailed Estimate
- Assembly Estimate
- Unit Estimate
Detailed Estimate determines the quantity and cost of each and every thing that is required for project.
Detailed estimating, also named as “Prime Cost”, “Bottoms Up” or “Grass Roots” estimates. As name indicates, these estimates requires a thorough and detailed analysis of all the tasks required to deliver a project or product.
It is obvious that these types of estimates are costly to produce because of the time and effort required. The accuracy however is the best among all the estimates.
These types of estimate is done when all information is available such as drawings, process sheets, planning etc. Each item is listed on estimate work-up sheet. Project is broken down into deliverable. Each deliverable has allocated time and cost.
After this WBS (work breakdown structure) is made which ensures that each deliverable is placed in right time so that project is completed in allocated time.
Following costs can be included in detailed estimate:
- Overheads etc.
An Assembly Estimate, rather than bidding for each individual components of the project, The Estimator bids the components in groups known as assemblies.
In order to fully understand what an assembly estimate is, lets consider simple example of an lightning switch assembly.
which contains following parts
- Single Gang-Box
- Single Pole Switch
- Cover Plate
- Two Wire Nuts
- Gage Wire
Lets now see how assembly estimating works in civil engineering.
Assembly method follows sequential approach defining how building will be constructed. One advantage of assembly estimate that two assemblies can be substituted in place of each other in order to quickly determine cost difference of two assemblies.
Assembly estimate is not as accurate as detailed estimate which makes it less costly too. An Assembly estimate is best when it is used as a budgeting tool in the planning stages of a project because the accuracy of the estimate is usually 15% +-.
Typical estimate of substructure and foundation estimate is shown below
Unit estimates are prepared by by multiplying unit area of the building by cost per unit area and then adjusting the price to compensate for differences in building height, length of the building perimeter, and other building components.
Unit estimates are sometimes refereed to as square-foot estimates as most common area unit used is square foot. These types of estimates are “educated guesses” as not much accuracy is achieved.
Unit estimates are common when there is uncertainty about the amount of work to be done.
Unit estimate has two parts
- price per unit of work involved for different phases of project
- Quantification of actual work which is completed on site.