Cost-Plus Contracts

The builder tells you the renovation is too difficult to price accurately as a fixed lump sum. There may be hidden conditions behind walls and under floors, and the design will continue to evolve. A cost-plus contract is proposed instead.

That can be a legitimate commercial model. But it changes the homeowner's risk in a fundamental way. Another guide introduces the choice between fixed price and cost-plus contracts; this guide focuses on how cost-plus pricing actually works in practice.

With cost-plus, the most important number may not be the estimate written at the beginning. It may be the formula that determines what you ultimately have to pay.

What is a cost-plus contract?

Under a cost-plus arrangement, the final price is based on the actual cost of carrying out the work plus an agreed builder's fee or margin. Unlike a conventional fixed-price contract, the final amount is not fully known when you sign.

South Australian law permits cost-plus pricing, subject to statutory requirements. Under the Building Work Contractors Act 1995 (SA) and its Regulations, the builder’s margin on a cost-plus domestic building contract is generally limited to 15% of the actual cost of the work. A margin exceeding the prescribed limit may breach statutory requirements and affect the enforceability of the excess. Homeowners should verify the current prescribed limit, as regulations may be amended from time to time.

Why use cost-plus?

Cost-plus can make sense where scope is genuinely difficult to price in advance, such as complex renovations, older buildings, uncertain existing conditions or highly bespoke work.

It can avoid pricing a large contingency into a fixed sum, but it shifts more price risk to the homeowner.

The trade-off is cost certainty

You may know the method for calculating the price without knowing the final price. A budget, estimate or target is not the same as a contractual cap unless the contract expressly creates one.

Understand what counts as 'cost'

Does cost include subcontractor invoices, materials, builder-employed labour, supervision, plant, travel, waste, insurance, administration or overheads?

The margin can only be understood once you know the cost base to which it is applied.

Read the margin with the cost definition

A lower percentage is not necessarily cheaper if the recoverable cost base is broader. Do not focus only on the margin percentage; read the entire pricing mechanism.

Estimates are not guarantees

An estimated total can be commercially useful, but the contract should tell you whether it is simply an estimate, a target, a budget or an enforceable maximum.

Where prices are estimates or subject to change, South Australian statutory requirements concerning warnings and fair and reasonable estimates can apply.

Transparency is fundamental

Because the homeowner is paying actual costs, the ability to see and verify those costs is central. The contract should make clear what records the builder provides and when.

Consider invoices, timesheets, subcontractor claims, credits, discounts and supplier rebates.

Set a reporting rhythm

Do not wait until the project is nearly complete to discover the budget is nearly exhausted. Regular reports can show costs incurred, committed costs, forecast cost to complete, margin and revised forecast.

Cost-plus does not eliminate variations

The agreed scope still matters. Changes can affect both the forecast and completion date. Keep written records of scope changes so everyone understands why the project cost is moving.

Control owner decisions

Repeated upgrades still accumulate and the margin may apply. Selections should still have budgets and deadlines.

Time can become a major cost driver

Depending on the cost definition, additional labour, supervision, plant and site establishment can feed directly into the amount payable. Programme management and delay responsibility are therefore particularly important.

Consider budget controls

Depending on the project, parties may consider target budgets, notification thresholds or approval requirements before significant expenditure is incurred. Cost-plus need not mean an uncontrolled blank cheque.

Cost-plus record keeping

Original estimate or target budget.

Definition of recoverable costs.

Agreed builder's margin.

Subcontractor and supplier invoices.

Builder labour and supervision records where chargeable.

Plant and equipment costs.

Credits, discounts and rebates.

Committed costs not yet invoiced.

Forecast cost to complete.

Running total.

Questions to ask before signing

Why is cost-plus proposed?

What exactly is included in 'cost'?

What margin applies and to which categories?

Is the quoted total an estimate, target or cap?

What evidence of actual cost will I receive?

How often will the builder report against budget?

How are discounts and credits treated?

How are labour and supervision charged?

Can approval be required before significant expenditure?

What happens if the forecast materially exceeds the estimate?

The key lesson

A cost-plus contract is not inherently good or bad. It is a different allocation of price risk.

It can be appropriate where genuine uncertainty makes a fixed price artificial, but it requires much greater understanding of the cost definition, margin, reporting system and budget controls.

Before You Sign

This guide provides general educational information for South Australian homeowners. It is not personalised legal or financial advice. Your position depends on your particular contract, lender requirements, documents and project circumstances. Before You Build is about understanding the bargain before it becomes difficult to change. A contract review can identify the obligations, risks and document inconsistencies that deserve attention before construction begins.

Before You Build. One Signature. One Chance.