How to Write a Contractor's Design Portion (CDP) Performance Specification

What is a contractor's design portion and how does JCT treat it?

A contractor's design portion is a defined part of the works that the contractor designs as well as builds. Under the JCT Standard Building Contract the employer's design team issues Employer's Requirements, the contractor answers with Contractor's Proposals, and a CDP Analysis prices the portion.

The term comes from the JCT family of contracts. In the Standard Building Contract the employer's design team prepares the main design, and a defined part of the works is carried out as a Contractor's Designed Portion. The recitals describe that portion, and the contract documents then include three items that belong to it: the Employer's Requirements, which say what the portion must do; the Contractor's Proposals, which are the contractor's response; and the CDP Analysis, which breaks down the part of the contract sum that relates to the portion. The Intermediate Building Contract with contractor's design and the Minor Works Building Contract with contractor's design use the same vocabulary for the same purpose. Check the recital and clause references in the edition and form in use.

The standard of the contractor's design obligation for the portion depends on the contract form and any amendments. Many forms frame it as reasonable skill and care rather than fitness for purpose, so check the executed contract rather than assuming. The specification writer does not set that standard. It is set by the contract, and the specification has to work within it.

Why the architect writes the requirements

The CDP exists because some elements are best designed by the people who manufacture and install them. A curtain wall supplier knows its mullion sections, its glazing bead and its thermal break far better than a generalist design team does. The Employer's Requirements capture what the client and the design team need from that element without pre-empting the specialist's solution. They are also the document against which the employer will later argue that a proposal fails, so they carry real weight in a dispute.

A CDP does not remove the architect from the element. The design team normally still sets the external appearance, the grid, the interfaces with adjacent work and the performance targets, and it reviews the contractor's design documents against those requirements. Where the architect stops and the specialist starts is a design responsibility decision, and it should be recorded in the design responsibility matrix before the specification is written.

What goes in a CDP performance specification?

A CDP performance specification states the scope and boundary, the interfaces, measurable performance criteria, the design inputs, the standards to follow, the deliverables and their dates, the testing and acceptance regime, and the warranties and insurance.

A CDP specification is a performance specification with a boundary. It has to say what the specialist designs, what it does not design, what it receives from the design team, what it must achieve and what it must hand back. The eight headings below cover the content that causes most of the disputes when it is missing.

Scope and boundary first

Start the clause with the boundary because every other heading depends on it. Define the portion by system and by location, and say what stays with the design team. A useful test is whether a specialist could mark the extent of the CDP on a plan without asking a question. If the answer is no, the clause cannot yet be priced fairly, and the first tender query will be a request for the drawing the clause assumes exists.

Performance criteria with standards behind them

Every performance criterion needs three parts: the value, the standard or test method that measures it, and the acceptance limit. "Fire resistance of 60 minutes" is incomplete. "Fire resistance of 60 minutes for loadbearing and integrity, classified to BS EN 13501-2, evidenced by a classification report for the installed configuration" can be priced and checked. Where a requirement comes from the Building Regulations, a fire strategy or a planning condition, name the source so the specialist knows it cannot be traded for cost.

Design inputs and what the specialist can rely on

Say which information the design team supplies and which the specialist must verify. If the structural engineer provides slab edge deflections and fixing zones, state them. If the specialist must confirm wind loading for the site, say that too. Silence on inputs becomes an argument later about who carried the risk of an assumption, and that argument sits in the gap between the Employer's Requirements and the Contractor's Proposals.

Deliverables, submission programme and acceptance

Tie each deliverable to a date that precedes the decision it informs. Calculations and general arrangement drawings are due before the order is placed. Shop drawings and samples are due before manufacture. Test evidence is due before installation starts. Name a limited number of review rounds, and require the contractor to programme the design period so that review time is visible. Check how the contract documents in use treat comments on the Contractor's Design Documents, and align any status codes the practice uses.

Which packages are typically contractor-designed?

The most common CDP packages are curtain walling and cladding, roofing and waterproofing, fire stopping, structural connections, and mechanical, electrical and sprinkler installations. Each is chosen because the specialist holds the detailed product knowledge.

No list of CDP packages is fixed. The design team and client decide package by package, and the choice follows how much design the architect has done, how bespoke the element is and where the specialist knowledge sits. The packages below turn up on most larger projects.

Two further points apply to every package. First, the scope of a CDP should be a coherent system. Splitting a facade between two designers on either side of a gasket line creates exactly the interface problems the CDP was meant to avoid. Second, check that the package is genuinely something the contractor can design. Elements that depend on aesthetic judgement, such as a signature stair balustrade, are often better designed in detail by the design team and made by a specialist to the drawn design.

For mechanical and electrical work the same principles apply, but the volume of design left to the contractor is usually larger, which makes the interface and coordination clauses more important. The MEP specification guide covers how that split is normally drawn.

How much should the architect design before handing over a CDP?

Design far enough that every acceptable proposal would share the same scope, interfaces and performance. Fix what is visible, shared or already decided, and leave the system, sections and fixings to the specialist.

The architect has to design far enough that the requirements are unambiguous and the interfaces are fixed. The difficulty is that a performance specification written too loosely invites the cheapest compliant answer, and one written too tightly takes the design back from the specialist while leaving the responsibility with them. The way to find the right level is to ask what has to be true of every acceptable proposal.

What the design team should fix

Fix anything that is visible, anything that crosses a package boundary and anything the client has already decided. That normally includes the grid and module, the external appearance and finish, the performance targets, the setting-out datums, the interface locations and the fire strategy. If the design team is vague on these, the specialist designs them by default, and the architect then reviews a design that the Employer's Requirements did not constrain.

What to leave open

Leave open the choice of system within the performance envelope, the section sizes, the fixings, the manufacturing method and the supplier. Allowing several acceptable systems keeps tender prices competitive. If the design depends on one product or one tested configuration, say so in the specification and accept that the package is then closer to a prescriptive one. The beginners' guide explains how the three specification types trade control for freedom.

Early specialist input

Where a package is complex, consulting a specialist before tender helps the design team set realistic performance values and spot interface problems. The requirements still need to be written so that other tenderers can respond. A specification that quietly names one specialist's system as the only solution is a proprietary specification, and should be written as one.

Where does design responsibility go wrong with a CDP?

Responsibility goes wrong where the boundary, an interface or an input is left unstated, where drawings contradict the performance clause, and where review comments start directing the design.

Most CDP problems come from the places where the specification is silent or contradicts the drawings. The pitfalls below are the ones that cause cost and delay on live projects. They are also the ones a pre-issue technical review should look for.

Reviewing is not designing

When the contractor submits its design documents, the architect or contract administrator reviews them against the Employer's Requirements. Comments should say where a submission does not meet a stated requirement or interface. Comments that say how to redesign the system risk taking design decisions on behalf of the specialist, and they invite the argument that the design team directed the solution. Keep review language factual: refers to the requirement, identifies the gap, asks for a revised submission.

The specification and the drawings disagree

A drawn detail that shows a specific bracket, glazing bead or cavity barrier inside a CDP package is a prescriptive instruction on top of a performance requirement. The specialist will ask which governs. Either remove the detail from the design team drawings and mark it "indicative, by CDP", or accept that the detail is part of the requirements and cannot then be changed without an instruction. Decide this explicitly for every drawn detail in a CDP zone.

Unassigned interfaces

Each interface between a CDP and another package needs an owner, a stated tolerance and a deliverable. A facade package that meets a roofing package at the parapet is a classic gap. If neither specification says who draws the junction, nobody does, and it surfaces on site as a variation request. State the owner in both packages, and cross-refer so the wording matches.

Insurance, warranties and collateral warranties

The employer is relying on the contractor's design, so the contract particulars normally call for professional indemnity cover for the design and may set a limit of liability. Take legal or insurance advice on the exact wording for each appointment. Echo the requirements in the specification section so that a specialist who tenders to the specification alone still sees them. Where the specialist is a sub-contractor, the employer may also want a collateral warranty. That is a legal document for the employer's legal adviser, and the specification only needs to say that one is required and when.

Building safety and statutory duties

Designers carrying out contractor-designed work have their own statutory duties, including those under the Building Regulations for work in England. The Part 2A duty holder guide explains who is a designer for these purposes, and the principal designer guide covers the coordination role. On a higher-risk building the CDP elements that govern fire and structural safety cannot be left unresolved until after the building control approval application, because the BSR needs to see how they will comply. What your specification must show at Gateway 2 covers what that means in practice.

• ✓Every CDP package has a stated boundary, extent and owner in the design responsibility matrix.
• ✓Every interface has a named owner in both adjoining specification sections.
• ✓Every performance criterion has a value, a standard and an acceptance test.
• ✓No drawn detail inside a CDP zone contradicts the performance requirement.
• ✓Every deliverable has a date that precedes the decision it informs.
• ✓Insurance, warranty and as-built requirements are stated in the specification section.

Questions technical directors ask about CDP specifications

What is a contractor's design portion?

A contractor's design portion (CDP) is a defined part of the works that the contractor, usually through a specialist sub-contractor, designs as well as builds, while the employer's design team designs the rest. In the JCT Standard Building Contract the CDP is identified in the recitals and the contract documents include the Employer's Requirements, the Contractor's Proposals and the CDP Analysis.

What is the difference between a CDP and a design and build contract?

In a design and build contract the contractor takes responsibility for completing the design of the whole of the works. With a CDP the employer's design team keeps design responsibility for everything outside the named portion, and the contractor is responsible only for the part described in the Employer's Requirements.

What standard of care applies to contractor-designed work under JCT?

JCT contracts allocate the standard of design care in the contract itself, so the answer depends on the form and edition. Many forms frame the contractor's design obligation as reasonable skill and care rather than fitness for purpose, so check the executed contract. The specification should not try to alter the standard of care, and any change belongs in the contract amendments, advised by the employer's legal adviser.

Does reviewing a contractor's design submission transfer design responsibility to the architect?

No, not by itself. The contractor stays responsible for its design, and a comment or acceptance from the architect does not normally take that responsibility over. The risk is that a comment goes further than reviewing against the Employer's Requirements and starts to direct the design. Limit review comments to compliance with the requirements and the interfaces the design team controls.

Which parts of a building are typically contractor-designed?

Common CDP packages are curtain walling and other cladding systems, roofing and waterproofing systems, structural steel connections, fire stopping and compartmentation details, mechanical and electrical installations, sprinkler systems, lifts, and fixing systems for facades and stairs. The right list depends on the project, the procurement route and how far the design team has developed each element.

How detailed should a CDP performance specification be?

Detailed enough that two specialists reading it would design systems with the same scope and the same performance, and no more prescriptive than the design intent needs. State the result, the standard it is measured against and the evidence required, then fix the visual and interface decisions the design team must control.

Where Avoice fits

A CDP works when the boundary, the interfaces and the evidence are written down once and kept consistent with the drawings. That is a comparison and governance task as much as a drafting one.

In Avoice the office master holds the practice's standard CDP clauses as a governed library, with the headings in section 02 built in, so a facade or roofing package starts with scope, interfaces, inputs, deliverables and acceptance already present rather than copied from the last job. Practice rules can require that every performance clause carries a value and a standard before the section is issued.

The pre-issue Technical Review compares the specification with the schedules and drawings, which is where drawn details that contradict a CDP performance clause and unmatched interface wording tend to show up. Construction Admin then carries the package into the build: submittals for the contractor's design documents, RFIs on interfaces and tracked changes to the specification.

Software can compare, flag and record. The decision about where the design team stops and the specialist starts remains a design responsibility decision for the practice and its client.