02 May 2017
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What is a specification?

According to the Shorter Oxford English Dictionary a specification is "a detailed description of the dimensions, construction, workmanship, materials etc., of work done or to be done, prepared by an architect, engineer, etc."

What's included and what's not included in a specification?

A specification is a document that describes in words what cannot be visualised or explained on a drawing or model. This document can be incredibly wide-ranging - covering the establishment of the site, the type of contract to be used, the performance criteria of the asset, the quality of the systems and products, which standards are applicable and how they should be executed, and even the products to be used.

Specifications do not include information on cost, quantity or drawn / visualised information so need to be read in conjunction with documents detailing quantities, schedules and drawings.

What types of specification are there?

Specifications are not only used in construction - they are also used in a range of other industries; from the aerospace, oil and gas and automotive industries to manufacturing.

The type of specification can relate to the project or the procurement route.

Specifications will vary depending on the stage to which the design has been developed: performance (open) specifications will require further design work, while prescriptive (closed) specifications draw on an already complete design.

A specification is a document that describes in words what cannot be visualised or explained on a drawing or model.

What are the advantages of each type of specification?

A very prescriptive specification at tender stage will ensure the client has a high degree of certainty about what will be delivered.

A performance specification gives suppliers more discretion, for example, to suggest innovative solutions and more cost-effective ways of working, potentially saving money in the long run.

What kinds of projects are best suited to which type of specification?

Performance specifications tend to be used on projects where typical, straightforward kinds of buildings are required.

Prescriptive specifications are typically used to deliver more complex buildings or where a client's requirements may not be known or assumed by suppliers and therefore the extra certainty will be important. Prescriptive specifications are also used where a very specific end result is required (for example, a supermarket chain will require a branded, uniform, kind of building - here the building type is likely to be straightforward but the client likely to insist on a prescriptive specification).

In reality, many projects will use a combination of specifications - both performance and prescriptive. Design aspects are likely to be specified prescriptively, less critical items (such as lifts) by performance only.

Deciding which approach to take involves weighing up who is likely to achieve best value for each particular element on the project - the client, designer or contractor.

How should specifications be structured?

The structuring of specifications will vary from project to project but should reflect the work packages on a particular project and any sub-contracts. This structuring should make it easier for contractors to price a job and give a more accurate tender.

The use of a standard classification system, such as Uniclass 2015, is encouraged as it should remove any potential for confusion or ambiguity.

When are specifications produced?

The production of specification documents should happen in tandem with design work - with ever greater level of detail added as the design progresses. At tender stage, the specification serves as an essential reference guide for contractors looking to price up a job. Leaving specifications until the last minute - when production information is being prepared - is not to be advised.