Last edited 29 May 2019

Contractor's design portion

The contractor’s designed portion (sometimes referred to as 'contractor's design portion' or CDP) is associated with JCT construction contracts. It is an agreement for the contractor to design specific parts of the works. The contractor may in turn sub-contract this design work to specialist sub-contractors. This should not be confused with design and build contracts where the contractor is appointed to design the whole of the works.

Until 2005, the JCT Standard Building Contract had a contractor’s designed portion supplement for use where the appointed contractor was required to design specific parts of the works. This provision has now been incorporated into the main contract, but a separate sub-contractor design agreement is available where the main contractor is to design specific parts of the works and a sub-contractor is to design part or all of the sub-contract works.

The client’s requirements for contractor's design will generally be set out in the tender documents as ‘Employer’s Requirements’ in response to which the contractor will submit ‘Contractor’s Proposals’.

Often, the portion of design left to the contractor is the detailing work undertaken by specialist manufactures and installers. The main contractor has responsibility for managing the procurement of its specialist sub-contractor’s design in an orderly fashion and to an agreed programme so it can be approved by the client’s design team (who within their conditions of engagement avoid liability for specialist design).

The main contractor also has to ensure that various specialist sub-contractors under its control co-ordinate their emerging designs with each other. For instance the services sub-contractors need to know both the grid system and position of fixings for a suspended ceiling and likewise the ceiling sub-contractor needs to know details of plant above ceilings which require maintenance access. To carry out this management function 'design managers' have become a regular addition to the main contractor’s project staff.

The main contractor may be given the responsibility for ensuring that its sub-contractor’s design is submitted for and obtains building regulations approval. The client’s design team will usually submit specialist design to the planning authorities when necessary to satisfy planning conditions since it is generally the design team’s responsibility to obtain full planning consent.

Design consultant’s need to judge how far to take their design before handing over its completion to specialists. Furthermore it must be made abundantly clear exactly what the specialist has to design within the work section. A performance specification must describe quality both in terms of function and form, bearing in mind that contractors tendering in competition will respond with the cheapest option available to meet the specification.

The use of a sub-contractor design approval procedures to improve what was tendered has been the source of back door variations and many contractual disputes in relation to cost and/or delay. On the other hand it is a waste of designers’ resources and time to develop the design of systems that may or may not be used. Closed specifications reduce options in the specialist market and this is not in the best interests of the client.

It is sensible for the main tender documents to ask for a sub-contractor design programme to be submitted by the main contractor within a certain period after the contract has been placed. This should track specialist design, the approval process, manufacture and installation and fit into the overall contract programme. It is reasonable for the design team to insist on an even workload of sub-contractor drawing submittals so it can balance its resources in relation to the comment and approval process.

The introduction of building information modelling (BIM) is likely to have a profound effect on contractor's design. If at the time of the main contractor's appointment, a fully-integrated and collaborative building model has been prepared, it is unlikely that this will simply be handed over to the main contractor.

The model is more likely to remain under the management of the client's design team. Whoever is managing the model will be the party that has to manage the collective input of specialist designers. The main contractor’s role in this process could therefore become redundant, which is not satisfactory under the current terms of contractor's design agreements because it effectively means the designers would be controlling activities of the main contractor’s sub-contractors.

However, the main contractor carries the risk of a fixed price and meeting the contract completion date. For this and for programme reasons, contractor's design in the way that we know it now could become a thing of the past. In any case the advent of BIM is likely to alter the timing of the inclusion of specialist design so it takes place in the pre-construction design stage rather than in the construction stage.