Specifications are written documents that describe the materials and workmanship required for a development. They do not include cost, quantity or drawn information, and so need to be read alongside other contract documentation such as quantities, schedules and drawings. Likewise, written information about materials and workmanship should not appear on drawings or in bills of quantities, instead they should refer to the appropriate clauses in the specification.
Specifications vary considerably depending on the stage to which the design has been developed when the project is tendered, ranging from performance specifications (open specifications) that require further design work to be carried out by the contractor, to prescriptive specifications (closed specification) where the design is already complete and no choices are left to the contractor.
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 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, 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. An exception to this might be a repeat client such as a large retailer, where a specific, branded end result is required and so whilst the building type is well known, the specification is likely to be prescriptive.
Most projects will involve a combination of performance and prescriptive specifications. Items crucial to the design will be specified prescriptively (such as external cladding) whilst less critical items are specified only by performance (such as service lifts).
- 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 iteratively alongside the design, and not left until the preparation of production information. By tender they should describe every aspect of the building in such a way that there is no uncertainty about what the contractor is pricing.
Aspects of the works are generally specified by:
- 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).
Specifications should be structured according to work packages mirroring the separation of the works into sub-contracts. This makes it easier for the contractor to price and so may result in a more accurate tender. The Common Arrangement of Work Sections (CAWS) includes 300 different work sections, reflecting the range of specialists and sub-contractors in common use. Proprietary specifications (such as the National Building Specification (NBS)) follow the Common Arrangement of Work Sections (CAWS) and are continually updated. Subscribing to such a service ensures that all clauses and standards referred to in a specification are up to date.
NB This system is currently undergoing considerable change, with CAWS being incorporated into Uniclass, Uniclass being replaced with Uniclass2 and SMM7 being superceded by the New Rules of Measurement (NRM). NRM uses its own system of indexing. See CAWS or New Rules of Measurement for more information.
The development of Building Information Modelling (BIM) can allow the creation of a building model populated with specification information meaning that there is no need to prepare a separate specification.
 Find out more
 Related articles on Designing Buildings Wiki
- Building information modelling.
- Bill of quantities.
- Common Arrangement of Work Sections (CAWS).
- Insulation specification.
- New Rules of Measurement (NRM)
- Output-based specification.
- Outline specification.
- Performance specification.
- Specification basics.
- Standard Method of Measurement (SMM7).
- Value management techniques.