Fixed Price vs Cost-Plus Building Contracts: What Australian Homeowners Need to Know

When your builder hands you a contract, the first question isn't "what does it say?" — it's "what type of contract is this?" The contract type determines how much price certainty you have, who bears the risk of cost blowouts, and how your builder is incentivised to work.

Fixed Price Contract

A fixed price contract sets a lump-sum amount for the completed home. This is the most common contract type for residential KDR in Australia and is used by most volume and project builders.

What it means in practice: The builder takes on the risk of their labour and material costs. If steel goes up 15% mid-build, that's (theoretically) their problem, not yours.

The catch: "Fixed price" contracts in Australia almost always contain variation clauses that allow the builder to charge extra for:

• Changed site conditions (soil, slope, drainage)
• Client-requested changes to design or materials
• Prime Cost (PC) and Provisional Sum (PS) items that come in above the allowance
• Delays caused by the client, council, or utilities

The contract price is fixed only within the scope defined in the contract. Get your soil report done, pin down all PC items, and minimise provisional sums before signing.

Cost-Plus Contract

Under a cost-plus (or "time and materials") contract, you pay the builder's actual costs plus a fixed fee or percentage margin (typically 15–25%).

When it makes sense: Complex custom homes, heritage renovations, or situations where the scope genuinely cannot be defined upfront. A good custom builder will often prefer cost-plus because it's honest about the unknowns.

The risk: You bear all cost risk. If the builder is slow, inefficient, or materials cost more than expected, you pay. Without rigorous oversight and an independent quantity surveyor reviewing invoices, costs can balloon unpredictably.

For standard KDR: Avoid cost-plus unless you have a trusted relationship with the builder, a strong QS overseeing every invoice, and a detailed scope of works.

Understanding Prime Cost and Provisional Sum Items

These are the two biggest sources of variations in fixed-price contracts:

Prime Cost (PC) Items are specific products where the final price isn't known at signing — typically fixtures like tapware, tiles, appliances, or lighting. The contract includes an "allowance" (e.g., $3,000 for kitchen taps). If you choose taps that cost $5,500, you pay the $2,500 difference.

Provisional Sums (PS) are estimates for work where the scope isn't fully defined — often things like rock excavation, retaining walls, or stormwater connections. These are guesses, not fixed prices. A contract with many high-value provisional sums is essentially a cost-plus contract in disguise.

What to do: Before signing, ask your builder to minimise provisional sums wherever possible. Lock in actual quotes from subcontractors for things like earthworks. Set maximum allowances for PC items and insist that any selections above allowance require your written approval before ordering.

The HIA and MBA Contracts

Most residential builders in Australia use the Housing Industry Association (HIA) or Master Builders Association (MBA) standard contract. These are industry-standard documents that are broadly balanced but lean slightly toward builders on variation rights.

Key clauses to understand in an HIA contract:

• Clause 14 (Variations) — Your builder must give you a written variation notice. You have the right to refuse client-requested variations but not site condition variations.
• Clause 17 (Delays) — Defines what counts as an extension of time. COVID-era supply chain delays established precedent that builders can claim extension of time for materials shortages.
• Clause 23 (Dispute Resolution) — Requires mediation before litigation. Know your state's building dispute body (NSW: Fair Trading, VIC: DBDRV, QLD: QBCC).

What to Do Before You Sign

1. Have a solicitor or building contract specialist review the contract before signing — typically $500–$1,500 but worth it.
2. Get your geotech report done first, then insist the builder quote to the actual soil classification.
3. Ask for a schedule of all PC and PS items with their allowances. If the total of PS items exceeds 5% of the contract value, push back.
4. Negotiate a sunset clause — if the builder hasn't started within a specified timeframe, you can exit the contract and recover your deposit.