Approximate quantities
A development of the elemental cost plan of estimating construction project costs, approximate quantities form part of an approximate quantities cost plan which represents a first attempt to measure defined quantities from drawings (or to ‘take them off’ from a building information model).
An approximate quantities cost plan is part of the iterative cost planning process and is a development of the elemental cost plan. Unlike the elemental cost plan (in which the cost of elements is broken down from the overall construction cost, based on the experience of the cost consultant and known costs of similar completed projects), the approximate quantities cost plan is a first attempt to measure defined quantities from drawings. In effect, it is a costed approximate bill of quantities.
The approximate quantities method is regarded as being more accurate and reliable than other estimating methods in showing where costs are distributed. In particular, it draws to the attention of designers those elements of the design that are standard and those that are not and which, as a consequence, may be more expensive. In effect it is a priced approximate bill of quantities.
But unlike the elemental method, the basis of the approximate method of estimating is the grouping together of diverse, combined items under a rate (known as a composite rate) which is calculated by combining the individual prices of the components in the group. Because the items grouped together have the same dimensions (whether in metres or square metres, or perimeter and area), it allows those items to be measured at the same time.
For example, under the approximate quantities method, items such as roofs can be grouped with soffits and ceilings; this would not occur in the elemental plan.
The approximate quantities cost plan can form a solid base for an effective value engineering exercise. However, it should be accompanied by a schedule of assumptions made and perhaps a cash flow projection. A pre-construction whole-life cost plan may also be prepared.
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