A specification is a document, addressed to a contractor, which describes requirements for a construction project. It sits alongside, and is linked to, drawings that show the physical layout of the project. The specification adds information about the materials and products to be used, and about the workmanship and procedures that are to be followed. There are two main kinds of specification In a prescriptive specification, the designer has made all the detailed decisions and the contractor simply executes the work. In a performance specification, the outcome of the work is specified by the designer, and the contractor is free to select suitable products and methods. In either case, specifications should be clear, concise and unambiguous. The specification is used to obtain prices for the work during the tender stage, and also to instruct the contractor during the preparation and construction phase. With a performance specification, the tenderers should submit their proposals for how the specification will be fulfilled, as well as their price.

Who should prepare the specification?
The designer is responsible for the preparation of the specification. On many smaller projects there may be only one professional adviser, usually an architect or building surveyor. On most larger projects there will be a number of design disciplines – architect, structural engineer, services engineer, landscape architect, etc. Each designer should prepare the specification for the types of work for which they have responsibility.

Timing
Specifying is an integral part of the design process and can be carried out most easily and effectively if done concurrently with preparation of the drawings. Decisions on specification start to be made early – the client’s brief often contains specific instructions and preferences, and other firm decisions are prompted by the obtaining of statutory approvals for the project. The majority of firm decisions on products and workmanship should be taken before completion of detailed design. The specification should be completed before inviting tenders.

Decision making
Specification is the most detailed level of design. An outline specification may accompany an outline design proposal, but a specification is usually thought of as a document that sets out the designer's intentions in some detail. Whether a specification is prescriptive or by performance, it reflects all the important decisions made during the evolution of a project. It should therefore be backed up by appropriate research, based on current standards, regulatory requirements and manufacturer's information.

Recording and retrieving information
Along with the drawings, the specification document records the designers' decisions. This is best done methodically and in a well-structured manner, typically using a classification system recognized by the construction industry (such as Common Arrangement or Uniclass). Proprietary master specification systems, such as those published by Twyford Bathroom and NBS, use these systems. It should always be easy for the design team to enter information, and for the contractor to retrieve it. Master specification systems provide an environment, either paper or software based, where decisions can easily be recorded.

Be precise
When writing a specification, be decisive and clear. The purpose of writing a specification is to give useful information, and there is little point in producing a specification that fails to do this. The contractor needs precise information before ordering products and carrying out the work. Specify explicitly. The use of language such as ‘to approval’ reduces the value of the specification as a working document and can significantly increase the specifier's liability.

Be clear about what you expect the contractor to do (performance specifications).
In a fully prescriptive specification, the designers have, in theory at least, specified all aspects of the work in detail. The specification conveys this information to contractors for pricing and execution. A performance specification moves some of the design responsibility to the contractor. The specifier must define the performance that the completed building is required to achieve, for example the sound or thermal insulation of a wall. The contractor is frequently requested to submit proposals as how the requirements will be met. For a successful outcome, those requirements must be clear and precise. To request the contractor to 'submit proposals' against a vague or ill-defined requirement, in order to avoid having to specify that requirement, carries the risk of those proposals being inadequate and under-priced.

Talk only to the contractor
People outside the contract include, e.g. tenderers, subcontractors, authorities, and separately engaged contractors. The contract, of which the specification is a part, is between the employer (or client) and the contractor. The specification is issued on behalf of the employer to the contractor, and contractually cannot bind other organizations. So references to these third parties, and the design team, should be appropriately worded.

Do not repeat the contract or legislation
It is a general principle of documentation that you should say something once, and in the right place. So if for example the contract specifies how work should be supervised, or that materials used should be new, then do not repeat this in the specification. Similarly if the law requires that water fittings comply with the relevant act, there is no need to say so in the specification. Compliance with building regulations also follows this principle.

Presentation
Specifications can be large documents, densely packed with information and, no matter how elegantly arranged and written, they can be heavy reading. Good presentation can make them much easier to use. Cheese-paring on presentation can all too easily become a false economy. In printed specifications it is useful to create page breaks where a contractor may need to split the document to copy and send on to sub-contractors.

Co-ordination
Specification information can be given in several places – the specification, drawings, bills of quantities or various types of schedule. Careful thought should be given to how this is done. It is recommended that full specification information is normally given only in the specification. Greater predictability and freedom from discrepancy will usually result if drawings, quantities and schedules identify the different kinds and qualities of work, but do not aim to specify them. Duplicating some of the detailed specification on drawings or schedules can lead to the contractor working from partial or out-of-date information.

Bills of quantities
Where bills of quantities are used, a full description of work may be given in the specification, if a precise and unique cross-reference is given in the bills of quantities. In this way, a full and precise specification can be deemed to be included in the bills, whilst concentrating the essential information in the specification document.

Master specification systems
National master specifications such as those produced by Twyford Bathrooms' SpecMaster and NBS are tools for the preparation of specifications. Like all tools they require technical expertise, and familiarity with proprietary software where this is used. Because they are used for a wide range of projects, their content is broad – much may go unused on a particular project. They also require editing – for example, most clauses contain options that need to be selected. Used well, specifications will be editorially, technically and contractually sound, and will have been produced more efficiently than equivalent specifications produced in any other way.