Specification for construction (2024)

Specifications describe the products, materials, and work required by a construction contract. They do not include cost, quantity, or drawn information, and so need to be read alongside other information such as quantities, schedules, and drawings.

Specifications vary considerably depending on the stage to which the design has been developed, ranging from performance specifications (open specifications) that require further design work to be carried out, to prescriptive specifications (closed specifications) where the design is already complete.

Performance (open) specifications describe the result that is required from particular items and leave it to the contractor or supplier to satisfy that requirement. In effect it requires the contractor or supplier to complete the design. The nature of the performance required may be defined by the desired outcome, or by reference to standards. For more information, see: Performance specification.

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.

For more information see: Prescriptive specifications.

Having a prescriptive specification when a contract is tendered gives the client more certainty about the end product, whereas a performance specification gives 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 and are well-known building types. Whereas prescriptive specifications are written for more complex buildings. For more information see: Prescriptive specifications.

They can also be used in combination. Items crucial to the design may be specified prescriptively (such as external cladding) whilst less critical items may be specified only by performance (such as service lifts).

Key to deciding whether to specify a building component prescriptively or not, is considering who is most likely to achieve best value when selecting an item, the client, the designers or the contractor:

• Large clients may be able to procure certain products at competitive rates themselves (for example the government).
• Some designers may have particular experience of using a specific product (although some clients may not allow designers to specify particular products as they believe it restricts competition and innovation and may relieve the contractor of their liability for 'fitness for purpose').
• The contractor may be best placed to specify products that affect buildability.

Specifications should be developed alongside the design, increasing in level of detail as the design progresses.They should not be left until the preparation of production information.

The first stage in the development of a specification is the preparation of an outline specification. An outline specification is a brief description of the main components to be used in construction. They should be described in sufficient detail to allow the cost consultant to prepare some approximate quantities. For more information see: Outline specification.

By the tender stage (when prices are sought from potential suppliers), they should describe every aspect of the building in such a way that there is no uncertainty about what is required.

Aspects of the works are generally specified by product or by workmanship:

• Products: by standard, a description of attributes, naming (perhaps allowing equivalent alternatives) or by nominating suppliers.
• Workmanship: by compliance with manufacturers requirements, reference to a code of practice or standards, or by approval of samples or by testing.

It should be possible to verify the standard of products and workmanship by testing, inspection, mock-ups and samples, and documentation such as manufacturer's certificates.

Specifications should be structured according to work packages, mirroring the separation of the works into sub-contracts. This makes it easier for suppliers to price and so may result in a more accurate tender.

A standard classification system should be followed, such as Uniclass.

See also:

• Functional specification.
• Output-based specification.
• Performance specification.
• Prescriptive specification.

• Bill of quantities BOQ
• Commissioning specification.
• Construction Specifications Canada CSC.
• Construction Specifications Institute CSI.
• Final specifications.
• Performance specification.
• Prescriptive specification.
• Specification guidance for construction.
• Tender documentation for construction projects.
• Schedule of work for construction.
• Technical specification.
• Outline specification.
• Output-based specification.
• Green Guide to Specification.
• Schedule.
• Common Arrangement of Work Sections.