You may have known very well about performance and conformance specifications. While the former informs the supplier about the outcome/output that they must deliver, the latter tells them the technical aspects of the product: dimensions, materials, etc. But are they enough for a contractor to construct a building?

Look at this design.

The drawing alone can be considered as a conformance specification for the contractor. Can they send you a detailed quotation solely based on this drawing? I think they can’t. Though this drawing is great, but lots of information is unknown by both the buyer and the contractor:

• Whether the wall will be made from bricks or woods?
• What kind of materials is needed to make doors and window frames? Wood, aluminum, steel or plastics?
• …

In fact, in Design-Bid-Build project, where the designer of the construction is independent of the building contractor, another type of specification is needed: the prescriptive specification.

A prescriptive specification typically contains 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.

It looks like this:

A prescriptive specification is used together with design specification so that a contractor can know what exactly they must do. From these documents, they can quote their work accordingly.