Cost Plus Contracts In AU Residential Construction: Common Disputes

What it is

A cost plus contract pays the builder the actual cost of carrying out the work plus a margin for overhead and profit. The margin is set as a percentage of cost, a fixed fee, or a combination. The builder does not carry the price risk of material movement or productivity variance. The owner does.

In Australian residential construction the cost plus form is heavily restricted. Section 13 of the Domestic Building Contracts Act 1995 (Vic) limits cost plus contracts to defined situations: where the work involves renovation, restoration or refurbishment of an existing building and it is not possible to calculate the cost of a substantial part of the work without doing some of it, or where the work is reasonably estimated to cost 1 million dollars or more (raised from 500,000 dollars in 2017). Victoria also requires a cost plus contract to contain a fair and reasonable estimate of the likely total cost.

NSW does not impose the same statutory restriction in the Home Building Act 1989 (NSW), but the disclosure obligations and consumer protection provisions apply, and the standard HIA and MBA cost plus templates are written to handle the typical risk allocation.

The shape of the disputes

Cost plus disputes follow a small number of repeating patterns. Builders and owners who understand these patterns before signing avoid most of the trouble.

Dispute one: the overhead and profit rate

The contract sets the margin (commonly 15 to 25 percent of cost). The dispute is whether the margin compounds onto certain categories of cost. Three flashpoints:

Subcontractor invoices. Does the margin apply to the full subcontractor amount, or only to the labour content of the subcontractor invoice? Most cost plus contracts apply it to the full invoice. Owners sometimes argue it should only apply to labour to prevent margin on margin.

Site supervision. Is the supervisor's wage charged as a direct cost (and therefore margined) or covered inside the percentage? The answer depends on the wording of the cost definition clause. Builders who do not look at this clause line by line find themselves either undercharging supervision or being challenged in tribunal.

Statutory charges and council fees. Is the margin applied to a 15,000 dollar council contribution? Most contracts exclude statutory charges from the margin calculation. If the contract is silent, dispute follows.

The Victorian and NSW templates require the margin treatment to be clearly disclosed. A vague margin clause is grounds for an owner to dispute every claim.

Dispute two: what counts as a justified cost

The owner has the right to know that the cost being charged is real. Common challenges:

Receipts and invoices not produced for materials taken from the builder's stock.

Subcontractor invoices that exceed market rates and lack quotes for comparison.

Charges for plant and equipment owned by the builder, billed at hire rates that exceed market.

Wages for the builder's family members or directors charged at trade rates without timesheets.

The cost is justified if it is reasonable, actually incurred and supported by source documents. Owners who insist on monthly cost reconciliation against source documents avoid most of these disputes. Builders who keep tight cost records, identify materials drawn from stock, and apply consistent plant hire rates published in advance defend the position easily.

Dispute three: audit rights and access

Standard residential cost plus templates give the owner the right to inspect cost records on reasonable notice during and after the build. The dispute is usually about scope: is the right to inspect the builder's general ledger, or only the cost records for this job? The answer in well drafted templates is the cost records for this job, including supplier invoices, subcontractor invoices, plant records and timesheets.

Where the contract is silent, the position is murky. In Victorian Civil and Administrative Tribunal (VCAT) decisions, tribunals have consistently ordered builders to produce job specific source documents to support the cost claim. A builder who refuses access in a residential cost plus dispute often loses the claim.

Dispute four: the upper limit (or absence of one)

A cost plus contract without an upper limit transfers all price risk to the owner. Some contracts include a guaranteed maximum price (GMP) which sets a cap, with the builder absorbing cost overruns above the cap. Disputes arise where:

The GMP applies only to certain components and the owner did not realise it.

The builder claims the GMP should be adjusted up for variations the owner requested.

The cost is approaching the GMP and the builder claims variations to keep work moving.

Victoria requires the cost plus contract to contain a fair and reasonable estimate of the total amount the builder is likely to receive. That estimate is not a cap, but it is a representation. A builder who exceeds the estimate by a large margin without a documented cause is exposed to a misleading conduct argument under the Australian Consumer Law.

Dispute five: variations on a cost plus contract

A variation in a cost plus contract changes the scope. The cost of the variation flows through the same cost plus mechanism, so why does variation paperwork still matter? Two reasons. The owner needs to authorise the change in writing before it is built, or they can refuse to pay. And the variation can change the GMP or the estimate, which has consequences if either was disclosed under section 13 of the Domestic Building Contracts Act 1995 (Vic) or under contract.

Where the cost plus model still works

Despite the dispute load, cost plus is the right contract for some residential work. Heritage restoration, complex renovations where structural unknowns sit behind walls and high end custom homes where the owner wants real time cost control all suit cost plus. The condition is the relationship: cost plus depends on owner trust in the builder's cost records and the builder's discipline in keeping them.

The construction industry's elevated insolvency rate, sitting around 26 percent of all Australian business failures in the year to March 2025, has pushed some builders to favour cost plus for higher risk jobs because it removes the material cost shock that triggered many of those failures. The trade off is the owner now carries that risk, and the dispute pattern shifts accordingly.

Practical defence position

A residential builder running cost plus contracts in Australia keeps job specific cost records every month, applies the margin consistently across categories disclosed in the contract, produces source documents on request and updates the cost forecast against the disclosed estimate at every progress claim. An owner under cost plus reviews the cost claim against source documents monthly, raises queries inside the claim period and confirms variations in writing before they are built. Both sides win when the paperwork keeps pace with the build. Both sides end up at NCAT, VCAT or the relevant tribunal when it does not.