Fixed Price vs Cost Plus for a High-End Custom Home in Victoria — Which Is Better?

For most high-end custom homes in Victoria, a fixed price contract is the right default, and cost plus is only worth considering when the scope genuinely cannot be defined yet — because in Victoria cost plus is not a free choice. It is only available in limited circumstances, the main one being a contract above $1 million. Below that threshold the question does not arise for most homeowners, and above it the real decision is not "which contract type" but "how much of this scope is actually defined."

That distinction is where most of the advice online goes wrong. The usual framing is risk appetite: fixed price means certainty, cost plus means flexibility, pick your temperament. That is not how it works on a $4–6 million build. The contract type does not decide who carries the risk of an undefined scope. The documentation does. The contract only decides who pays for it when it surfaces.

What the law actually allows in Victoria

Both contract types sit inside the Domestic Building Contracts Act 1995 (Vic), and the Act constrains them more than most people expect.

A major domestic building contract is required for most work over $10,000. Below that, you are outside the regime. Every custom home is well inside it.

Cost plus is restricted. It is not available as a general option for major domestic building work. The circumstances in which it is permitted are limited, and the one that matters for custom homes is a contract price above $1 million. (This threshold was lifted from $500,000 on 1 August 2017.) If a builder offers you cost plus on a $900,000 job, that is a question worth asking them directly.

The deposit is capped. Five per cent if the contract price is $20,000 or more. Ten per cent below that. There is no luxury exemption — a $5 million contract still has a 5 per cent deposit ceiling.

Progress payments are staged and capped. For a contract covering all stages, the maximums are base 10 per cent, frame 15 per cent, lock-up 35 per cent, fixing 25 per cent. If a payment schedule in front of you front-loads beyond these, it is not a negotiating position; it is non-compliant.

From 1 December 2026, cost escalation clauses are capped. Under the Domestic Building Contracts Amendment Act 2025, on contracts of $1 million or more a cost escalation clause can add no more than 5 per cent to the contract price. This is new, and it changes the calculus — it puts a hard ceiling on the "fixed price with a rise and fall clause" structure that has been common at the top end since 2022.

Insurance changed on 1 July 2026. Contracts signed from that date fall under Home Warranty insurance rather than Domestic Building Insurance: cover applies to eligible work over $20,000, the maximum is $400,000 per home (up from $300,000), and — the substantive change — it is no longer limited to the builder having died, disappeared or become insolvent. It can respond where work is incomplete, defective or non-compliant and the builder fails or refuses to fix it.

Those are the rails. Neither contract type lets a builder outside them.

What a fixed price contract actually fixes

A fixed price contract fixes the price of the work that is described in the contract documents. It does not fix the price of the house you are imagining.

The gap between those two things is where fixed price builds blow out, and it has three usual components:

Provisional sums. An allowance for work whose scope is known but whose cost is not yet established — site works and excavation are the classic case. If the allowance is wrong, you pay the difference.

Prime cost items. An allowance for an item not yet selected: tapware, appliances, stone, fixtures. On a high-end build these are the single most common source of overrun, because the allowance is usually set against a mid-range product and the selection is made against a luxury one.

Variations. Changes to the contracted scope. On an architect-led home these are often not the client changing their mind but documentation resolving itself late.

A fixed price contract with thirty provisional sums and a thin prime cost schedule is a cost plus contract wearing a suit. The number on the front page is not a price; it is a starting position. This is the thing to interrogate before you compare two tenders — a lower fixed price with larger allowances is frequently the more expensive house.

What cost plus actually costs

Cost plus pays the builder documented costs plus an agreed margin, usually a percentage. The honest argument for it is real: on a project where scope genuinely cannot be defined — an unusual site, a heritage fabric whose condition is unknown until opened up, a design still resolving — a fixed price can only be priced by loading contingency into it. You pay for risk that may not materialise.

The honest argument against it is also real, and it is structural rather than moral: a percentage margin means the builder's fee rises with cost. That does not make a builder dishonest. It does mean the contract contains no financial mechanism pushing cost down, so every mechanism that does must be imported — open-book reporting, a cost plan to measure against, a defined approval process above a threshold, and, in practice, the client's own time and attention.

Cost plus also shifts the cash flow profile. You are reimbursing as costs are incurred, not paying at defined stages, so the finance structure has to suit it. Lenders are often less comfortable with cost plus for exactly this reason, and that is worth testing with your broker before the contract type is settled rather than after.

How to actually choose

The question is not "which is safer." It is "how much of this is defined?"

Choose fixed price when the documentation is complete enough to price — full construction drawings, a resolved specification, engineering done, and selections either made or carried at realistic allowances. If that is true, fixed price transfers the risk you are paying to transfer, and you should expect the provisional sum and prime cost schedule to be short.

Choose cost plus when the scope genuinely cannot be defined and you have the appetite and availability to govern it — and then insist on the governance: a cost plan at the start, open-book reporting against it, written approval above an agreed threshold, and a margin that is fixed in dollars or capped rather than open-ended percentage.

Treat a hybrid as the honest middle. Fixed price on the defined work, a clearly scoped cost plus or provisional allowance on the genuinely unknown portion — most often early site works — converted to a fixed sum once the unknown is resolved. This is common on complex eastern suburbs sites and it is usually the structure that reflects reality.

The test we apply before quoting: if we cannot describe what is behind a provisional sum in a sentence, it is not an allowance, it is a guess, and it does not belong in a fixed price contract pretending to be one.

Questions worth asking before you sign either one

• How many provisional sums are in this contract, and what is the total value of them as a percentage of the contract price?
• What is each prime cost allowance based on? Name the product.
• Is there a cost escalation clause? From 1 December 2026, on a contract of $1 million or more it cannot add more than 5 per cent — is that reflected?
• What is the deposit? It cannot exceed 5 per cent.
• Does the progress payment schedule sit within the prescribed stage maximums?
• On cost plus: what exactly is "cost"? Is the builder's own labour, supervision, site establishment and preliminaries inside the margin or charged as a cost?
• On cost plus: what is the reporting cadence, and what do I receive — a summary, or line items reconciled to invoices?

Where we land

We write fixed price contracts for most of our work, because by the time we are quoting we have usually been involved through documentation and the scope is defined well enough to price honestly. Where it is not — a site with genuine unknowns below ground — we would rather scope that portion separately and convert it than inflate a fixed price with contingency the client cannot see.

A builder who will tell you which parts of your project they cannot yet price, and why, is giving you more useful information than one who hands you a single confident number.

Crave Build is a registered domestic builder (Domestic Builder – Unlimited, DB-U 102065) building architect-led custom homes and knockdown rebuilds across Melbourne's eastern suburbs. This article is general information about Victorian building contracts, not legal advice. For your own contract, take advice from a construction lawyer.