Fixed Price Versus Lump Sum Contracts In AU Residential Construction

What it is

In Australian residential construction the words "fixed price" and "lump sum" are routinely used as if they mean the same thing. They do not. A lump sum contract is a contract for a single agreed price for a defined scope of work. A fixed price contract is one where the agreed price is not subject to variation for the specified causes, usually rise and fall in cost of labour and materials. Every fixed price contract is a lump sum, but not every lump sum contract is fixed price.

The distinction matters because it changes who pays when costs move during the build, and because the standard residential templates from HIA, MBA and the state regulators handle the two differently.

Lump sum: one price for a defined scope

A lump sum contract sets a single price (the "contract sum" or "contract price") for a defined scope of work. Variations to scope are added or deducted under the variation clause. Standard NSW and Victorian residential templates default to lump sum.

Section 7 of the Home Building Act 1989 (NSW) requires the contract price for residential building work above the prescribed threshold to be stated in the contract or be capable of calculation under the contract. That language allows for lump sum, lump sum plus PC and PS, or schedule of rates, provided the calculation method is clear.

The risks a builder carries under a pure lump sum:

Scope risk. If the documents do not show a detail and the building standard requires it, the builder pays.

Productivity risk. If trades are slower than estimated, the builder pays.

Site risk to the extent not excluded. Unexpected ground conditions, latent services, and access constraints all sit with the builder unless qualifications are written into the contract.

The risks the owner carries:

Variation cost. Anything the owner adds, changes or upgrades during the build.

PC and PS adjustments. Final cost of items only allowed for in the contract.

Fixed price: lump sum plus no rise and fall

A fixed price contract goes one step further than lump sum. It adds a clause confirming the contract price is not adjustable for rise and fall in the cost of labour, materials, statutory charges, or fluctuations in subcontractor pricing. The standard HIA New Homes Contract and the standard MBA residential contracts give the parties a choice to include or exclude rise and fall.

The Domestic Building Contracts Act 1995 (Vic) and the Home Building Act 1989 (NSW) do not mandate fixed price across the board. They mandate disclosure of how the price works. A builder who labels a contract "fixed price" and then claims a rise and fall variation has misled the owner and is exposed under Australian Consumer Law misleading conduct provisions enforced by the ACCC and state fair trading regulators.

In practice, almost all volume residential builds in Australia are fixed price. The owner is borrowing against the contract price for finance, and the lender wants to know the maximum exposure. The builder accepts the price risk in exchange for the volume and the scheduled deposit and progress payments.

Where the terms cause disputes

Site costs

Most fixed price contracts contain a site costs allowance based on a soil report and survey done at tender. When the actual conditions differ (rock encountered, fill depth greater than expected, asbestos in existing slab) the builder claims a variation. Owners often resist on the basis that they signed a fixed price. The contract usually permits it, but the dispute is about whether the cause was excluded from the fixed price scope. Clear qualifications at tender prevent most of these disputes.

Statutory charge increases

Council fees, water authority charges and energy connection charges sometimes rise between contract signing and the relevant approval. Standard contracts allow these to be passed on as variations. A builder who agrees to "absolutely fixed" pricing without that carve out is paying out of margin.

Material cost shocks

Between 2021 and 2024 timber, steel and plasterboard costs jumped sharply. Fixed price builders absorbed those increases on jobs already signed. The construction industry insolvency rate sat at around 26 percent of all Australian business insolvencies in the year to March 2025, a position the industry had not seen for a decade. Fixed price exposure is one of the most cited triggers.

Provisional sums and prime cost items

A truly fixed price contract still adjusts for PC and PS items. The fix is on the works the builder controls. The owner picks tapware later and the difference between the PC allowance and the actual cost flows through to a variation. This is not a breach of the fixed price; it is how PC items work and how they must be disclosed under the Victorian and NSW regimes.

Practical risk allocation table

The risk under each contract type can be summed up:

Pure lump sum without rise and fall protection

Builder carries: scope, productivity, material cost movement, labour rate movement, site conditions to extent not excluded.

Owner carries: variations they request, PC and PS adjustments.

Lump sum with rise and fall clause

Builder carries: scope, productivity, site conditions to extent not excluded.

Owner carries: variations, PC and PS, material and labour cost movement above thresholds in clause.

Fixed price (lump sum without rise and fall)

Builder carries: everything except clearly excluded items.

Owner carries: variations they request, PC and PS.

Cost plus

Owner carries: actual cost of all works plus builder's margin.

Builder carries: very little price risk. Use restricted in Victoria under section 13 of the Domestic Building Contracts Act 1995 (Vic) to contracts where it is genuinely impossible to estimate cost or where work is renovation of existing buildings, with disclosure obligations.

What to write into the contract

Builders who want a defensible fixed price position write three things into the contract: a clear scope reference to the priced documents (drawings and specifications by revision number), a clear list of qualifications and exclusions, and a clear schedule of PC and PS items with cost basis disclosed. Owners who want a defensible fixed price position make the builder name what is excluded and what triggers a variation, and they keep all changes in writing.

Calling something "fixed price" without doing this work is a slogan, not a contract position. Section 8 of the standard HIA and MBA templates is where the price structure is set and that is the clause to read line by line before signing.