Episode 209:

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I am your host Maria Skoutari and this week we will be talking about what we mean by the term Contractor Design Portion. Todays’ episode meets PC5 of the Part 3 Criteria.

And make sure to stay until the end for today’s scenario.

So today, continuing on the theme of contractors design responsibility, we’ll be covering a topic that crops up again and again in UK contracts, exam questions and, live projects: the Contractor’s Design Portion, often shortened to CDP.

If you’re working mostly under JCT building contracts, you’ll know that the Contractor’s Design Portion sits in an awkward middle ground between a fully traditional arrangement, where the design team carries almost all design responsibility, and full design and build, where the contractor takes on the majority of the design of the works. It’s one of those areas where it’s very easy for assumptions to creep in and for design responsibility to be muddled, which is exactly what clients, insurers and the courts don’t want.

So in today’s episode we’ll cover three big themes:

• What Contractors Design Portion actually is, and how it differs from design and build or straightforward traditional procurement.
• The JCT contracts where you’ll come across Contractors Design Portion and how it is structured in those forms, including JCT Design & Build.
• And the practical and professional issues around scope, coordination, liability and how you, as an architect or contract administrator, should manage it on live projects.

So to start off, what is the Contractor’s Design Portion, what do we mean by that definition: 

Under the JCT, the Contractor’s Design Portion is the part of the works that the employer asks the contractor to design, in addition to building them. It is not the whole project, it is specific elements or packages where design responsibility shifts to the contractor, often because specialist knowledge, proprietary systems or performance‑based solutions are needed.

Examples you’ll often see in practice include:

• Specialist cladding or curtain wall systems.
• Sprinkler or fire suppression systems.
• Lifts, escalators and certain MEP systems.
• Specialist structures, like long‑span steel trusses or feature staircases and so on.

The design responsibility for elements like that is carved out in the tender documentation and then crystallised in the contract as the contractor’s design portion. The contractor may then sub‑contract that design to specialist sub‑contractors, but as far as the employer is concerned, the main contractor remains responsible for that portion of the design.

Historically, JCT used a separate Contractor’s Designed Portion Supplement for the Standard Building Contract, but those provisions are now incorporated into the main standard form. The Intermediate Building Contract, however, still uses a discrete ‘with contractor’s design’ version and related sub‑contract forms. That evolution is worth remembering for Part 3 because it reflects how embedded CDP has become in traditional procurement.

Now let’s move onto how the Contractors Desing Portion is embedded into the different procurement routes:

Starting with traditional procurement, under this route, the employer appoints a design team who develop the design to a high level of detail, enough to tender a lump‑sum contract based on drawings, specification and, often, bills of quantities. The contractor’s role is fundamentally to construct in accordance with that design, with only limited design in temporary works or methods.

However, traditional projects increasingly require specialist input and proprietary systems that architects and engineers are not best placed to detail fully. To address this, traditional JCT forms allow certain elements to be identified as a Contractor’s Design Portion, giving the contractor design responsibility for those parts while the overall project still follows a traditional pattern.

By contrast, under JCT Design & Build, responsibility for completing majority of the design rests with the contractor, based on the Employer’s Requirements and the Contractor’s Proposals. In that model, the whole project is effectively a ‘contractor’s design portion’, albeit with careful distinctions around what is in the Employer’s Requirements and what is in the Contractor’s Proposals.

You will also see Contractor Design Portion concepts appearing in:

• The JCT Intermediate Building Contract with contractor’s design (ICD), used for traditional‑style projects of intermediate complexity where the contractor designs specified parts.
• And even the JCT Minor Works Building Contract with contractor’s design, where the contract is essentially a Minor Works form plus additional provisions for limited contractor design responsibility.

So the headline for exam purposes is that the Contractors Design Portion is primarily a feature of traditional and intermediate contracts, but the underlying idea, clearly defining which parts of the design sit with the contractor, runs right through to full design and build.

So let’s move on the specifics on where Contractors Design Portion appears in JCT Forms:

Starting with the Intermediate Building Contract with contractor’s design (ICD 2024), JCT’s own description states that it is for use where the appointed contractor is to design specific parts of the works, while the employer’s design team retains responsibility for the overall design. The employer must provide drawings and bills of quantities, a specification or work schedules to define the quantity and quality of work, and to set out the requirements for the contractor’s designed portion.

Now under the Minor Works contracts with contractor’s design, it is similar in concept but pitched at smaller, simpler projects, giving the employer a light‑touch way to delegate limited design responsibility to the contractor. Again, the employer’s documents describe the required performance and scope, and the contractor undertakes design for those identified parts.

On a Standard Building Contract, the Contractors Design Portion provisions effectively import design‑and‑build‑style responsibilities for those defined elements into the otherwise traditional framework. Commentary on these contracts notes that the CDP mechanism reproduces, for the designated parts, the key design responsibilities that a contractor would carry under a JCT design and build contract.

And then we have JCT Design and Build itself. Under the JCT Design and Build contract, the contractor’s core obligations include completing the design for the works and carrying out and completing the construction in a proper and workmanlike manner in accordance with the contract documents. The Employer’s Requirements set out what the client wants, and the Contractor’s Proposals explain how the contractor will meet those requirements, the contractor’s design documents then implement that.

So while the CDP is explicitly called out in the traditional and intermediate forms, it is helpful, both in practice and in exams, to recognise that JCT Design & Build represents the same idea taken to its logical full‑project conclusion, that the contractor is responsible for design, subject to important limits about the contents of the Employer’s Requirements.

Now, speaking of Employer’s Requirements, let’s zoom in on the documentation that makes the Contractors Design Portion work, because this is where a lot of risk actually sits:

For any contractor’s design portion, the employer’s team needs to define clear requirements at tender stage. On contracts like the Intermediate Contract with Contractors Design, JCT notes that the employer must provide drawings, specifications and other documents that both specify the quantity and quality of work and detail the requirements for the contractor’s designed portion.

In a design and build context, these appear as the Employer’s Requirements. Setting out the client’s needs, objectives, performance criteria and sometimes outline design, and the contractor responds with Contractor’s Proposals that show how those requirements will be met. Those two documents are then annexed to the contract and form the basis for the contractor’s design responsibility.

For limited Contractors Design Portions under traditional forms, the pattern is similar, just narrower in scope:

• The Employer’s Requirements (or equivalent description within the tender documents) define what the Contractors Design Portion element must achieve, such as fire performance, acoustic standards, durability, structural spans or interfaces with the rest of the building.
• And the contractor, often using specialist designers or manufacturers, produces proposals and later design documents that show how those performance and interface requirements will be met.

The architects role is twofold. First, they assist the client articulate robust, coordinated requirements, and second, they review the contractor’s design submissions for compliance, usually on a ‘reasonable skill and care’ basis, not by taking over design responsibility. That division is key because it links into professional indemnity insurance, duty of care and the limits of what ‘approval’ or ‘comment’ on contractor design actually mean.

Now, who is actually liable when something goes wrong with a Contractors Design Portion element:

Under traditional JCT forms with a contractor’s design portion, the contractor assumes design responsibility for those identified parts of the works, broadly comparable to its design responsibility under a design and build contract for the whole works. They are expected to exercise reasonable skill and care in that design, and, unless expressly amended, JCT does not impose a fitness for purpose obligation.

In the design and build form, JCT guidance and legal commentary emphasise that the contractor is responsible for completing the design for the works, but not for verifying the adequacy of design contained in the Employer’s Requirements unless that duty is expressly imposed. If an inadequacy is found in the Employer’s Requirements, and the contractor is not responsible for verifying their adequacy, then correcting that inadequacy is treated as a Change, potentially entitling the contractor to additional time and money.

Recent case law analysis has highlighted that amendments can significantly alter this balance, including shifting full design responsibility back onto the contractor, even for aspects originating in the Employer’s Requirements. The key lesson here is to always check what the contract actually says about design duty, including any bespoke amendments, and do not rely solely on the unamended JCT position.

For the architect, design responsibility remains with them for the non‑CDP portions of the works. They are not automatically responsible for the contractor’s design portion simply by commenting on or ‘approving’ their drawings. Standard wording and professional guidance make clear that their role is usually to review for compliance with the Employer’s Requirements and with their own design, not to warrant the contractor’s design.

However, there is a practical grey area, if the architect goes beyond that role, for example, by redesigning Contractors Design Portion elements, or by issuing instructions that effectively change the contractor’s solution, then they may start to assume additional responsibility. That’s why the way they comment on Contractors Design Portion submissions and the clarity of their appointment are crucial.

So let’s contrast how the Contractors Design Portion feels in practice on a traditional project with a contractor’s design portion versus a full design and build project:

Starting with the traditional project with Contractors Design Portion:

• The employer and their design team lead and coordinate the overall design.
• They define clear performance and interface requirements for the Contractors Design Portion elements in the tender documents.
• The contractor designs only those identified elements, often through specialist sub‑contractors, and is responsible for both the design and construction of that portion.
• The architect or contract administrator administers the contract, certifies payments, and reviews Contractors Design Portion submissions against the requirements and against the rest of the design.

In contrast, on a JCT Design & Build project:

• The contractor is responsible for completing the entire design, based on Employer’s Requirements and Contractor’s Proposals.
• The employer’s pre‑contract design team often becomes novated to the contractor after contract award, so their duty of care shifts.
• The contractor’s design duty is expressed as a reasonable skill and care obligation, unless amendments impose a higher ‘fitness for purpose’ standard.
• The employer’s representative focuses on monitoring compliance and managing the contract rather than detailed design development.

Conceptually, Contractors Design Portions is a targeted application of design and build principles within a traditional framework. For specific elements, the contractor takes on design risk in return for being able to control the design of those elements and often to integrate specialist suppliers more efficiently. That hybrid approach can unlock value, but it also creates interface risks, which is where good coordination, clear documentation and thoughtful procurement strategy come in.

So as architects or contract administrators, what should we actually be doing when a project includes Contractors Design Portion:

First, architects should assist the client to decide whether including a Contractors Design Portion is appropriate for the project in general. Legal and technical commentaries point out that Contractors Design Portion is most useful where the contractor or specialist has genuine expertise and control, for example in complex building services or proprietary façade systems. If the employer’s team is clearly better placed to design something, pushing it into the Contractors Design Portion purely to ‘transfer risk’ can be false comfort, especially if the Employer’s Requirements are vague.

Second, they should ensure the Employer’s Requirements, or equivalent Contractors Design Portion description, are robust. That usually includes:

• Defining performance criteria (fire, acoustic, thermal, structural, durability).
• Setting out required standards and statutory compliance, including building regulations and relevant guidance.
• Clearly describing interfaces with the rest of the design, for example, movement joints, fixings, tolerances and coordination with services.
• Identifying any specific approvals needed, such as third‑party certifications or warranties.

Third, during delivery, a structured process for reviewing the contractors design should be put in place. That might involve:

• A submission schedule for the Contractors Design Portion elements.
• Clear status codes for comments (for example, ‘no comment’, ‘proceed subject to comments’, ‘revise and resubmit’).
• Coordination workshops with the contractor and specialists to resolve interface issues early.

Professional literature emphasises that whoever manages the digital model or central design coordination must carefully integrate specialist designers’ input. If the design team keeps control of the model, they must manage specialist information without inadvertently taking on the main contractor’s design responsibilities.

And lastly, architects should keep an eye on changes to Contractors Design Portions scope. If the client or design team decides to reclaim some design responsibility, or if the contractor’s proposals go beyond the original scope, they may need to adjust appointments, insurances and the building contract itself to match that shift.

Now, let’s look more closely at risk, because Contractors Design Portion is fundamentally a risk‑allocation tool:

On the employer’s side, CDP can transfer certain design risks to the contractor, especially where the contractor is in the best position to control design and installation. That might reduce disputes about specialist design failures and can make pricing more realistic because the contractor can package design and installation together with their chosen supply chain.

On the contractor’s side, Contractors Design Portion increases risk because they are now responsible for getting the design right within the specified performance parameters, they must manage their specialist designers and ensure that their design integrates properly with the rest of the project. Insurers and legal advisers will usually be keen to ensure that the contractor’s design duty is limited to reasonable skill and care and that no unintended fitness for purpose obligations creep in.

For the architect, the Contractors Design Portion can be a double‑edged sword. It can reduce their design workload and liability in some areas, but it also introduces interface risks and the potential for misunderstandings about who is responsible for what. That is why the appointment, scope of services and PI insurance need to be aligned with the reality of Contractors Design Portion on the project.

And to conclude today’s episode, let’s look at a few pitfalls and good‑practice points that you can bring into both your exam answers and your day‑to‑day work:

Common pitfalls can include:

• Vague Employer’s Requirements for Contractors Design Portion elements, leading to ambiguous scope and disputes when the installed system does not meet the client’s expectations.
• Assuming that the Contractors Design Portions means ‘the architect is off the hook’, when in reality interface design and coordination remain critical responsibilities for the design team.
• Treating contractor design review as a box‑ticking exercise rather than a serious check against performance and interface requirements.
• Ignoring the effect of bespoke amendments that may expand or reduce the contractor’s design duty and thereby change risk allocation.

In terms of Good practice, this should look more like:

• Early identification of potential Contractors Design Portion packages as part of the RIBA Stage 2–3 development, so that the procurement strategy and Employer’s Requirements can reflect them properly.
• Clear, measurable performance criteria and interface descriptions in the tender documents for each Contractors Design Portion element.
• A structured system for reviewing contractor design submissions, with clear communication about what your comments mean and what they do not mean.
• And regular design coordination sessions involving the contractor, specialists and the design team, especially at the boundaries between CDP and non‑CDP elements.

Let’s sum what we ran through today:

• Contractor's Design Portion (CDP) assigns the contractor responsibility for designing specific elements like cladding or MEP systems, distinct from full design & build. It is featured in a number of JCT contracts, such as the Intermediate, Minor Works, and Standard contracts. As well as Design & Build which extends the concept project-wide via Employer's Requirements.
• Generally, robust requirements are essential when defining the contractors design, and tender documents must specify performance criteria, interfaces, and standards to avoid disputes. Architect reviews should be noted as being for compliance only not assuming design responsibility.
• Liability stays with the contractor for Contractor's Design Portion design under 'reasonable skill & care’ and the architects retain non-Contractor's Design Portion duties, but should be aware for interface risks.
• Best practice is to identify the Contractor's Design Portion elements early, use structured reviews and coordination and avoid vague specifications or assuming full offloading of the architects role.