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The primary function of approximate or preliminary estimating is to produce a forecast of the probable cost of a future project, before the building has been designed in detail and contract particulars prepared. In this way the building client is made aware of his likely financial commitments before extensive design work is undertaken.

Unit of accommodation method

Single-rate approximate estimating
This method is commonly used by national bodies such as the education and health services at the inception stage of construction. If a client has an amount of money to spend (a budget) then it would be possible to consider the likely number of functional units which can be provided. From experience, it might be found that the cost of providing a study bedroom in student accommodation is £20 000.

Using this figure an expenditure of £12 million would provide accommodation for approximately 600 students. On the other hand if the number of units is known, a budget cost (usually expressed as a cost range) can be calculated. Providing there are recent comparable data available, the unit method is useful where a simple and quick cost range is needed in the early stages. It is difficult, however, to adjust the costs for specific projects, in different locations, with varying ground conditions and so on.

Floor area method

The main reason for the popularity of the floor area method is its simplicity.There are few rules to remember and the cost per square metre is well understood by property developers. A proposed building is measured at each floor level (between inside faces of external walls); no deductions are made for internal walls, stairs or lift zones. Previous similar building costs are used by dividing the construction cost by the internal floor area. Adjustments can be made for location and inflation; but specification adjustments are much more difficult to estimate. Subjective judgements are made for size, shape, number of storeys, services, ground conditions and standard of finishes. A separate assessment should be made for external works, demolitions, incoming services and drainage which can be significantly different for similar buildings.

There are many buildings where the unit of accommodation method is impracticable; such as warehouse projects or open-plan offices. In these cases the superficial floor area method is found to be reliable with an accuracy of 10% to 15%. This method also works well with certain external works contracts such as concrete paving or macadam surfacing.

Sometimes contractors are asked to quote for building work using sketch drawings and a square metre price. It is unlikely that a contractor would risk signing a contract on this basis. First a clear scope of works would be needed together with a site survey and soil investigation report.The price must accurately reflect the amount and specification of works.

Building volume method

There are several methods, which use the volume of a building as the cost yardstick, but they are not widely used today. In some European countries, architects and engineers are familiar with building costs expressed as cubic metre prices. In Germany, there are publications, which list typical building costs in terms of their volume, and the procedure for calculating volumes is given in a DIN standard.




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