Last edited 12 Oct 2020

Prescriptive specification

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

For more information, see Specifications.

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

Prescriptive specifications typically contain detailed descriptions of the following components:

General requirements relating to regulations and standards.
The type of products and materials required.
The execution and installation methods required.

Prescriptive specifications give the client much more certainty about the end product when making investment decisions (such as when they appoint the contractor), and place a greater burden on the designer to ensure proper installation rather than the contractor.

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

Prescriptive specifications can also be used on projects where there is a very specific end result required; for example, where it involves a repeat client such as a large retailer where, even though the building type is often straight-forward, a specific and branded end result is required.

In fact, most projects will involve a combination of performance and prescriptive specifications, where items crucial to the design will be specified prescriptively (such as external cladding), while less critical items, or items requiring specialist design are specified only by performance (such as service lifts).

Key to deciding whether to specify a component prescriptively or not is considering who is best placed to select that component, i.e. who is most likely to achieve best value, the client, the designers, the contractor or suppliers.

NB an output-based specification defines only the outputs that are required from the project (that is, what it will enable the client to do), it does not attempt to address how those outputs might be achieved. For more information see: Output based specification.