Traditional or design and build: which route suits your project?

Four routes compared

Summarised from JCT's practice note, Deciding on the appropriate JCT contract 2024:

Traditional procurement on a house

On the traditional route the design is completed and documented first: drawings, a specification or a schedule of works, and the contract particulars. Contractors then tender against the same information.

JCT notes that many traditional contracts require the client to appoint a professional consultant as an independent contract administrator. That person issues instructions, values the work, certifies payments and decides extensions of time. See what a contract administrator does.

Changes during construction are possible, but JCT is plain that they usually cost money and time. A lump sum is only as realistic as the information it was priced on.

Design and build on a house

Under design and build you set out what you want in your requirements, and the contractor's proposals respond to them. Three points in JCT's note are worth reading before you choose it:

• JCT's Design and Build Contract has no independent contract administrator. You make the decisions yourself or appoint an agent to act for you.
• Where your requirements contain a conflict, the contractor's proposals prevail, subject to compliance with statutory requirements. The quality of your requirements decides the quality of the building.
• Tenders are harder to compare, because each design proposal has to be judged against its price.

JCT also notes that asking the contractor to take over your own designers, known as novation, can cause problems over who is accountable for the design.

There is a middle course. JCT's Minor Works and Intermediate contracts each come in a version with contractor's design, where the contractor designs an identified part of the work (a steel frame, say, or the heating system) inside an otherwise traditional contract. JCT is clear that this is not design and build.

Two-stage tendering

In a two-stage tender a contractor is chosen on a first-stage price, then works with your team under a pre-construction services agreement until it submits a definitive second-stage price. JCT's agreement for this is written for use with its Standard, Intermediate, Design and Build and Major Project contracts. It is not listed for Minor Works, so two-stage tendering belongs to projects larger or more complex than a typical house extension.

Management routes, in one line

In management contracting and construction management the work is let in packages under a managing contractor or construction manager, which suits large projects where an early start matters more than an early price.

The dutyholder roles follow the route

Whichever route you choose, CDM 2015 applies. Where more than one contractor will work on the project, regulation 5(1) requires the client to appoint a principal designer and a principal contractor in writing. On a home, if you make no appointment, regulation 7(2) makes the designer in control of the pre-construction phase the principal designer and the contractor in control of the construction phase the principal contractor. Under design and build, the contractor controls more of the design, so who is in control of what needs settling at the start. ClearCDM explains the domestic position.

What JCT asks you to weigh

Of the matters JCT's note lists, these tend to decide a home project:

• how much of the design you want to control, and who you want to answer for it;
• how firm the price needs to be before you commit;
• whether you will live in the house during the work, and any limits on access, noise or working hours;
• how likely you are to change your mind once work starts.

The contract follows from the route. See which building contract.

What this means for you

If you want to fix the design and choose between comparable prices, the traditional route gives you that, at the cost of time spent finishing the design before tender. If an earlier start, or handing the design risk to the contractor, matters more, design and build can suit, provided your requirements are thorough and someone checks the contractor's proposals against them. JCT notes that single point responsibility is, in practice, not often achieved.

The free procurement and contract chooser walks through the same questions. Parallel Architecture tenders projects and administers the building contract on the traditional route: see tender and contract administration.