Design and construct (D&C): a builder's risk primer

TL;DR

Design and construct (D&C) is the procurement model where the builder takes responsibility for both the design and the construction under one contract. Volume residential builders (Metricon, Henley, Coral Homes) operate D&C by default; custom builders pick it up when the client doesn’t have an architect or engineer in the loop. The headline risk transfer is design liability: in a construct-only contract the architect and engineer carry that risk; in D&C the builder does, and the builder needs professional indemnity (PI) insurance to back it. NSW’s Design and Building Practitioners Act 2020 is bringing this into sharp focus, with mandatory PI for registered building practitioners commencing 1 July 2026. Outside that, D&C still attracts a price premium because the builder is pricing buildability risk, design coordination, and consultant-management overhead into the lump sum.

What D&C is, and what it isn’t

Under construct-only (often called “design-bid-build” overseas), the client engages an architect to produce a design, an engineer to produce structural details, then puts the design out to tender and signs a build contract with the winning builder. The builder constructs to the issued design and the consultants stand behind their drawings.

Under D&C, the client engages the builder under a single contract. The builder either has in-house design staff (volume builders) or sub-engages the architect and engineer themselves. The builder warrants that the constructed building meets the brief; design errors and buildability issues sit with the builder, not the client.

A pure D&C contract is rare in residential. The most common forms are:

Volume builder D&C: client picks a floor plan from a catalogue, builder modifies it within “promotion” or “premium” limits, builder handles council approvals, builder constructs. The “design” the builder warrants is the builder’s design.
Custom D&C: client provides a brief or sketch, builder commissions the architect and engineer, builder constructs. The architect and engineer work for the builder, not the client.
Design-assist or novated design: client engages the architect for early design, then “novates” the architect to the builder at construction-contract execution. The builder takes on the design from that point. Common on bigger projects, rare in Class 1a residential.
Hybrid lump-sum with design responsibilities: a notionally construct-only contract that pushes some design risk to the builder via fitness-for-purpose clauses, performance specifications, or builder-prepared shop drawings. Read the contract carefully: the label “lump-sum” doesn’t mean construct-only.

What changes for the builder under D&C

Three things shift in the builder’s direction.

Design liability sits with the builder. If a detail fails (insufficient bracing, undersized beam, wet-area falls that pond), the client can’t sue the consultant; they sue the builder. The builder then has to pursue the consultant under the builder-consultant agreement. This is why the PI insurance requirement turns on.

Variations are harder to claim. Under construct-only, a client-instructed change is a variation at the builder’s day rate. Under D&C the client can argue the change was always required to meet the brief, and the builder absorbs it. Tight brief documentation, a documented scope of works, and clear variation procedures are the defences.

The lump-sum carries a design-risk premium. Volume builders price this into their margin; custom builders should too. A naive “construct-only price + same margin” approach to D&C underprices the risk and exposes the business when a design call goes wrong.

PI insurance: when D&C makes it mandatory

Most residential builders running standard lump-sum builds don’t carry PI; their construction works policy covers the physical asset and their public liability covers third-party injury. PI covers financial loss from professional advice, design decisions, and documentation errors, the things construction works won’t touch.

D&C work crosses into “professional advice” territory. The state framework matters:

NSW: PI is already mandatory for registered design practitioners under the Design and Building Practitioners Act 2020. From 1 July 2026, registered building practitioners (the licence most home builders hold) must also carry PI. A NSW builder doing D&C work after that date without PI is uninsured for design liability and out of compliance with their licence.
Victoria: building designers and surveyors must carry PI under the Building Act 1993 (Vic) framework. Builders doing construction-only work generally do not, but a builder doing D&C is taking on design responsibility regardless of label.
Queensland: QBCC mandates PI for specific licence classes (building certifiers, building designers, project-management-services licensees). A builder under a standard QBCC builder licence isn’t mandated, but again D&C work creates exposure that PI would cover.
Other states (WA, SA, TAS, NT, ACT): state requirements are lighter, but the contractual exposure is the same. A judgement against the builder for design failure doesn’t care whether PI was state-mandated.

PI policies operate on a claims-made basis: the policy must be active when the claim is lodged, not just when the work was done. Builders winding down a D&C book need to carry run-off cover for several years after their last D&C project, or stay exposed.

When D&C makes sense, and when it doesn’t

D&C makes sense when:

Client wants single-point responsibility and is willing to pay the premium for it.
Builder runs in-house design or has an established consultant team and can price the design risk accurately.
Project is repeatable (volume builder floor plans, standard duplex layouts) so the design risk is well-bounded.
Buildability and constructability are the dominant risks, and the builder controls both.

D&C is the wrong call when:

Client has a specific architect or engineer they want to use, but wants D&C labelled for procurement reasons. In practice the builder is signing a fitness-for-purpose warranty for someone else’s design.
The brief is loose (“contemporary modern feel”, “open plan”). Without a tight brief, scope creep is the builder’s problem, not the client’s.
Builder has no PI cover and is operating in a state framework that’s about to mandate it (NSW, post-1 July 2026).
Project is one-off and the builder isn’t pricing the design coordination overhead into the margin.

Common traps to watch for

Fitness-for-purpose vs reasonable skill and care: D&C contracts often include a “fitness for purpose” warranty (the building will achieve the client’s stated purpose). PI policies typically only cover the lower “reasonable skill and care” standard. Negotiate the warranty wording carefully; a fitness-for-purpose clause may not be insured.
Novated consultants and the “design freeze”: if you novate an architect, lock down the design at novation and treat post-novation client changes as variations. Without a freeze, every “minor update” the client requests becomes builder-borne design rework.
Cost-plus D&C: rare, but seen. The builder takes design risk without the lump-sum buffer to absorb errors. Avoid unless the consultant agreements are watertight and PI is in place.
Implied design risk in performance specifications: a contract that says “the builder shall provide a slab capable of supporting the kitchen design as it evolves” hides design risk in performance language. Read these clauses as if they were D&C clauses.

References

NSW Government, Design and Building Practitioners Act 2020. https://legislation.nsw.gov.au/view/whole/html/inforce/current/act-2020-007 (verified 2026-05-14).
NSW Fair Trading, Professional indemnity insurance for registered building practitioners. https://www.fairtrading.nsw.gov.au/ (verified 2026-05-14).
Victorian Building Authority, Insurance requirements for building practitioners. https://www.vba.vic.gov.au/ (verified 2026-05-14).
Queensland Building and Construction Commission, Licence categories and insurance requirements. https://www.qbcc.qld.gov.au/ (verified 2026-05-14).