The Contractor's Design Portion: value it by its own rules

The Standard Building Contract allows a Contractor's Design Portion. The contractor designs a defined part of the works, while the rest is designed by the employer's team in the usual way. It is a way of placing design responsibility for a discrete element with the party best placed to carry it, without turning the whole project into a design and build.

Because the CDP sits inside an otherwise traditional contract, it is easy to treat it as just another part of the measured works. It is not. It has its own defining documents, its own valuation basis and its own liability standard, and the surveyor who ignores those three things produces a certificate that reads tidily and reconciles badly.

What defines the CDP

Two documents define the Contractor's Design Portion. The Employer's Requirements set out what the employer wants the designed part to achieve. The Contractor's Proposals set out how the contractor intends to meet those requirements.

Together they establish the boundary of the CDP: what is inside it, what the designed part must do, and what the contractor has committed to deliver. Everything that follows on valuation and on liability runs from these two documents, so the first task on any CDP question is to read them and establish where the line sits.

Variations to the CDP have their own valuation basis

When the CDP changes, the change is not valued the way an ordinary measured variation is valued. Variations to the Contractor's Design Portion are valued under the CDP's own rules, using the CDP Analysis.

The CDP Analysis is the pricing document for the designed part. It is the basis against which a change to the CDP is measured and priced, in the same way that the priced measured works give the basis for valuing ordinary variations. Reaching for the general measured rates to price a change to designed work is using the wrong document.

The practical discipline is to keep the two streams apart. A variation that touches the CDP is priced against the CDP Analysis. A variation that touches the measured works is priced against the measured basis. Where a single instruction affects both, it splits, and each part is valued on its own footing.

Liability for the designed part

Design carries responsibility, and the CDP places that responsibility on the contractor for the part it designs. The standard is ordinarily one of reasonable skill and care, the standard a competent professional designer is held to, unless the contract states a higher duty of fitness for purpose.

That distinction matters, and it is a matter of what the contract says rather than what is assumed. Reasonable skill and care asks whether the contractor designed competently. Fitness for purpose asks whether the result actually works for its intended purpose, which is a harder duty to discharge. Which standard applies is read from the contract, not taken for granted.

For the surveyor, liability is not a valuation input on an ordinary certificate, but it frames how a defect or a shortfall in the designed part is handled. A problem in the CDP is a problem in work the contractor was responsible for designing, and that shapes the commercial conversation in a way it does not for employer-designed work.

Why separate lines matter

The single most useful habit on a CDP job is to keep the designed part on its own lines in the valuation.

• The CDP is valued against the CDP Analysis, so its lines need to trace to that document, not to the measured basis.
• Variations to the CDP are priced by CDP rules, so they belong with the CDP, not scattered through measured variations.
• Liability for the designed part sits with the contractor, so it helps to see at a glance which value relates to designed work.

Fold the CDP into the general measured works and all three of those distinctions disappear. The certificate still totals correctly, but it no longer shows which value is designed and which is measured, which variation was priced against which document, or where the design liability attaches. When a question arises later, the answer has to be reconstructed by hand.

The check on a CDP valuation

• Have you read the Employer's Requirements and the Contractor's Proposals to fix the boundary of the CDP?
• Are CDP variations valued against the CDP Analysis rather than the general measured rates?
• Is the designed part on its own lines, traceable to its own basis?
• Have you confirmed from the contract whether the liability standard is reasonable skill and care or fitness for purpose?

The CDP is a small idea with precise consequences. Value it by its own rules, keep it on its own lines, and the certificate stays honest about what is designed and what is measured.