Input specification vs output specification
Traditional specifications are ‘input specifications’. They are prescriptive, that is, they describe in detail the products, materials and work (the inputs) required by a contract. Typically, the development of the solution required by the contract is complete and no (or little) further development is required by the supplier. The specification is considered to be a ‘closed specification’.
Prescriptive input specifications give the client certainty about the end product when making investment decisions, but place a greater burden on them to ensure the product has been thoroughly considered, developed and described. They tend to be used where a very specific end result required.
Output specifications define only the outcomes (or outputs) that are required by the contract, that is, what it will enable the client to do. They are open specifications that do not attempt to address how those outputs might be achieved. For example, an output specification might require the provision of a space for teaching 30 primary pupils, but it would not specify the nature of the space, the types of doors and windows etc. The standard for what is required might be defined, for example, by reference to performance targets.
It is argued that output specifications can get the best value from the supply team by allowing them to adopt innovative solutions to the client's requirements and reduce whole-life costs rather than simply developing the client's pre-conceived solutions at the lowest price.
For more information see: Output-based specification.
[edit] Related articles on Designing Buildings Wiki
- Briefing documents.
- Feasibility studies.
- Final specifications.
- Insulation specification.
- Outline specification.
- Output specification
- Output-based specification.
- Performance specification.
- Prescriptive specification.
- Procurement.
- Service level specification.
- Specification.
- Tender documentation.
- Tender process.
Featured articles and news
We're expanding our collaborative mission by launching DB Intelligence, an exclusive market research advisory panel. Built environment professionals can now get paid to share their expertise on industry trends, products and services.
Panel members receive direct financial incentives for participating in research projects like short surveys, 1-2-1 interviews and focus groups. Register today to shape the future of the construction sector.
Building Control Independent Panel final report
A precis of a key report led by Dame Hackitt with full recommendations and link to the government response.
Guide to ISO 19650 for Architecture Firms (2026)
A user gives their low down.
A UK training and membership provider for mould remediation professionals.
Building Safety recap April, 2026
A short and longer run-through of the month, with links to further information and sources.
CIAT May 2026 briefing.
Independent NSI and BAFE study exploring how organisations are changing the way they buy fire safety services.
From medieval scribes to modern word art.
ECA welcomes crackdown on late payment and push for clean energy, whilst CIOB seek fixed cladding removal timeframes.
Cyber Security in the Built Environment
Protecting projects, data, and digital assets: A CIOB Academy TIS.
Managing competence in the built environment
ITFG publishes new industry guide on how to meet the ICC principles.
The UK's campaign to reduce noise pollution: Mythbusting, articles and topic guides.