Explanation

In construction, specifications are written documents that describe the materials and workmanship required for a development. They do not include cost, quantity or drawn information but need to be read alongside other contract documentation such as quantities, schedules and drawings.

Specifications vary considerably depending on the stage to which the design has been developed, ranging from performance (open) specifications that require further design by a contractor or sup-plier, to prescriptive (closed) specifications where the design is already complete when the project is tendered.

Prescriptive specifications give the client more certainty about the end product when they make their final investment decision (i.e. when they appoint the contractor), whereas a performance specification gives the contractor and suppliers more scope to innovate and adopt cost effective methods of work, potentially offering better value for money.

Typically, performance specifications are written on projects that are straight-forward, standard building types, whereas prescriptive specifications are written for more complex buildings, or buildings where the client has requirements that might not be familiar to contractors and where certainty regarding the exact nature of the completed development is more important to the client.

Performance specification has some disadvantages:

- Well-defined performance metrics are needed to ensure that the specified performance will achieve the desired outcome

- Require reliable, practical, economical tests of performance

- Evaluations are subjective and require additional time and effort to complete

Therefore, if a performance specification is used, the buying organisation will have to ensure that they are able to define and conduct tests on whether supplier's solution can deliver the desired out-come.