17 June 2020

The specification drives the look and feel of a project, and heavily influences its cost plan. New design review tools such as virtual reality (VR) place even more importance on illustrating the right products at the right time.

With this in mind, the RIBA Plan of Work 2020 (reproduced overleaf and highlighting the requirements for exchanging specification information) places emphasis on the role of the specification during the design and construction stages. The outline specification is a crucial design tool during stages 2 and 3, allowing specification decisions to be captured in a document that can be shared with and agreed on by all.

A key inclusion in the RIBA Plan of Work 2020 is the acknowledgement that the stage 4 information from the design team needs to be geared to manufacturing or construction for each building system, with this information being descriptive or prescriptive, depending on the approach.

With the former, a specialist subcontractor will pick up the design baton, completing the information for manufacturing and/or construction; with the latter, the design team will issue the information used on-site for construction. For descriptive specifications, design responsibility lies with the contractor, although this responsibility is likely to be passed on to a specialist subcontractor carrying professional indemnity insurance. On a design and build contract, design responsibility rests with the contractor, regardless of who designs: whether the design team or a specialist subcontractor.

A common misconception is that procurement drives this decision-making. This is not correct: specifications can be prescriptive or descriptive, regardless of the procurement route. Traditional contracts have had the ability to facilitate descriptive specifications, via the contractor design portion (CDP), for years. Similarly, prescriptive specifications can be used where a planning consent has stringent conditions relating to materials, or where the client wishes to ensure that specific design aspects will be provided as imagined by the design team, meaning that they are commonplace on design and build contracts.

To phrase this another way, the decision to use a prescriptive or descriptive specification is driven less by procurement and more by topics such as the value proposition. The descriptive-to-prescriptive journey on any project will vary. Some clients may have a range of products used throughout their portfolio of projects, which might be included in the project brief. Other clients may wish the design team to specify anything that is visually important, allowing the contractor to select concealed products. There is no right or wrong decision; however, it’s essential to recognize that the decision must be made at the outset of the project. This allows appointment documents and the building contract to be prepared accordingly, and procurement discussions to be framed appropriately. The RIBA Plan of Work 2020 has, however, been designed in a manner that helps ensure that the complexity of this crucial project interface is clear from the outset.

The specification drives the look and feel of a project, and heavily influences its cost plan.

Dale SInclair, Director of Technical Practice at AECOM

In some instances, descriptive specifications are used to clearly assign design responsibility to the contractor – specifying the fire, acoustic and durability criteria for partitions, for example. However, the design team might specify products agnostically, taking design responsibility but allowing the contractor to propose a supplier for certain products. For example, the architect may specify a certain type of brick that is referenced in the planning consent whilst allowing the contractor to select walls ties and lintels, or specify a specific partition build-up, allowing the contractor to propose an alternative.

For some aspects, a descriptive specification can be the key to unlocking design innovation. For example, by issuing design intent information and a descriptive specification, the design team allows the cladding contractor to propose a unique design proposition that meets the goals of a project cost-effectively. The other notable anomaly to the prescriptive specification process is the preparation of shop drawings. For example, a supplier might produce drawings for toilet cubicles or a roller shutter which aids manufacturing, and would be based on the design team’s prescriptive information. In many instances, it is not issued for comment.

Another important stage 4 decision is the point at which stage 4 information is produced. Design and build procurement may draw down stage 4 information for inclusion in the employer’s requirements, with residual stage 4 information being completed by the contractor’s design team or the novated design team. This interface needs to be considered carefully at stage 1. The client must decide whether the information issued by the design team will be descriptive or prescriptive, and whether this information will be produced before or after the building contract is signed.

More information

Dale hosted the first in our series of RIBA Plan of Work webinars. To watch this and other webinars on-demand looking at the topics in the RIBA Plan of Work 2020, visit :

To download the RIBA Plan of Work 2020 and the overview guide see: https://www.architecture.com/knowledge-and-resources/resources-landing-page/riba-plan-of-work

Download the 2020 National BIM Report: