The Victorian answer most people do not expect

In many states, fixed-price versus cost-plus is a genuine open choice. In Victoria it largely is not, and that is good news for home builders. Under the Domestic Building Contracts Act 1995 (Vic) and the Domestic Building Contracts Regulations 2017, cost-plus contracts for domestic building work are restricted to a narrow set of circumstances. For a standard new home or house-and-land build, your builder will - and in most cases must - use a fixed-price major domestic building contract.

A major domestic building contract is required whenever the work is valued over $10,000, and it carries mandatory consumer protections: a fixed contract price (or a clearly defined method of calculating it), implied warranties, deposit limits, and set progress-payment stages. That framework is designed to stop the open-ended budget blowouts that cost-plus can create.

• Over $10,000 of work = a major domestic building contract is required
• Fixed price (or a defined calculation method) is mandatory for most builds
• Implied warranties, deposit caps and set progress stages apply by law

What each contract type actually means

A fixed-price (lump-sum) contract sets a single agreed price for a defined scope of work. The builder carries the risk of their own pricing and productivity - if materials or labour cost them more than estimated, that is their problem, not yours, provided the scope does not change. Your budget certainty comes from the scope being locked down in detail before you sign.

A cost-plus contract is one where the final price cannot be determined at signing: you pay the actual cost of the work plus an agreed margin (a percentage or fixed fee). It shifts price risk onto the owner, which is exactly why Victoria limits it. Where it is permitted, the law still requires the contract to include a fair and reasonable estimate of the total you can expect to pay.

When cost-plus is actually allowed in Victoria

As the regulations stand, a cost-plus domestic building contract is only permitted in defined classes of work: projects reasonably estimated to cost $1 million or more; renovation, restoration or refurbishment of an existing building where it is genuinely not possible to calculate the cost of a substantial part of the work without first doing some of it; and domestic building contracts for public construction. For the typical Melbourne new home, none of those apply - so fixed-price it is.

If a builder proposes cost-plus for a standard build under $1 million, treat it as a red flag. A contract that breaches these rules is unenforceable by the builder, and at VCAT the builder may only recover their actual costs plus a reasonable profit where the tribunal considers it fair. Oversight of these contracts is moving from Consumer Affairs Victoria to the new Building and Plumbing Commission (BPC) from late 2026, but the restrictions themselves remain.

• Cost-plus allowed only for $1M+ projects, defined reno work, or public construction
• Any cost-plus contract must still include a fair, reasonable total estimate
• A non-compliant contract is unenforceable by the builder

How to protect your budget whichever applies

With a fixed-price build - which is what you will almost certainly sign - the risk is not the price, it is the scope. Vague "provisional sums" and "prime cost items" are where a fixed price quietly becomes variable. Pin down a detailed specification, challenge every provisional allowance (especially site costs), and get a soil report so the site-works figure is real rather than a placeholder. On a rare permitted cost-plus job, insist on an estimated cap, regular open-book accounting, and an independent quantity surveyor to review claims.

In every case, verify the builder's registration on the VBA register before you pay a deposit, keep the deposit within the legal cap, and have a building lawyer or experienced consultant review the contract. BuildPilot is live in Victoria and matches you only with registered Victorian builders who work to compliant fixed-price contracts.

• Lock down the specification; challenge provisional sums and PC items
• Get a soil report so site costs are real, not a placeholder allowance
• Check VBA registration and keep the deposit within the legal cap
• Have a building lawyer review the contract before signing