What Is a Lump Sum Contract?
A lump sum contract is a construction contract in which the builder agrees to complete a clearly defined scope of work for one fixed total price, agreed before work starts and unchanged regardless of what the job actually costs the builder to deliver. It is the most common way residential and commercial building work is priced in Australia once a project is fully designed, because it gives the client price certainty and gives the builder a firm figure to build a business around — provided the underlying quantities were measured accurately in the first place.
Key takeaways
- A lump sum contract sets one fixed total price for a defined scope, agreed before construction starts.
- The lump sum figure is built up from measured trade quantities, preliminaries, contingency and margin, then presented as a single number.
- Because the price is fixed, the accuracy of the original measured takeoff matters enormously — an under-measured job quietly erodes the builder’s margin.
- Provisional sums and prime cost items can still sit inside an otherwise lump sum contract for parts of the scope that aren’t yet fully defined.
- Changes to the agreed scope after signing are handled through a formal, priced variation, not by simply adjusting the lump sum informally.
What is a lump sum contract?
A lump sum contract — sometimes called a fixed-price contract — commits the builder to complete a defined scope of work for one agreed total figure, set out in the contract before construction begins. Unlike a cost-plus arrangement, the client doesn’t see or pay for the builder’s actual costs as the job proceeds; they simply pay the agreed lump sum, usually in progress payments tied to stages or claims.
This arrangement only works when the scope is genuinely well defined — a complete set of drawings, a specification and, on larger jobs, a priced Bill of Quantities, all locked in before the price is agreed. Without that level of definition, a builder has no reliable basis for committing to a single number, which is why lump sum contracts are typically used once design is finished rather than during early planning.
A lump sum contract is the dominant pricing method for standard Australian residential builds and much commercial work, precisely because it hands the client a firm figure to budget and finance against, while giving the builder full responsibility for managing the job to that figure.
In plain terms
A lump sum contract is one agreed price for the whole job, full stop. Whatever it actually costs the builder to deliver, the client pays the same figure they signed up for.
What a lump sum contract includes
A properly drafted lump sum contract is built on a defined scope of work, referencing the drawings and specification the price was actually based on, so both parties can point to exactly what was included.
- A defined scope — drawings, specification and any Bill of Quantities the price was measured against.
- The agreed lump sum total, often broken into a progress payment schedule tied to construction stages.
- Provisional sums and prime cost items for the specific parts of the scope not yet fully designed or selected.
- A variation clause setting out how any change to the scope will be priced and agreed after signing.
- Contract conditions covering practical completion, defects liability and dispute resolution.
How a lump sum price is built up
Behind every lump sum figure sits the same build-up as any other estimate — it’s only the way the number is presented to the client that changes. An estimator measures the trade quantities from the drawings, prices preliminaries for the site-wide running costs, adds a contingency for genuinely unforeseen risk, and applies a margin on top — the total of all of that becomes the single lump sum figure quoted to the client.
Because none of that detail is disclosed to the client the way it would be on a cost-plus job, the accuracy of the underlying takeoff matters enormously. If the measured quantities are wrong, the builder — not the client — absorbs the difference, since the contract price doesn’t move just because the real cost turned out higher.
| Item | Amount |
|---|---|
| Trade totals (from BOQ) | $410,000 |
| Preliminaries | $58,000 |
| Subtotal | $468,000 |
| Contingency (5%) | $23,400 |
| Margin (12%) | $58,970 |
| Lump sum contract price | $550,370 |
Advantages of a lump sum contract
For the client, a lump sum contract’s biggest advantage is certainty — the total cost is known before the first trade arrives on site, which makes financing and budgeting straightforward and removes the risk of an open-ended final bill. Progress payments can be planned against a fixed figure rather than an estimate that might move.
For the builder, a lump sum contract rewards efficient management: if the job is run well and costs come in under what was priced, the builder keeps the difference as extra margin. It also simplifies contract administration — there’s no ongoing obligation to disclose actual costs the way a cost-plus arrangement requires.
Risks of a lump sum contract
The flip side of price certainty is that the builder wears the risk if the job costs more than priced — which is exactly why accurate measurement and realistic contingency at tender stage matter so much more on a lump sum job than on a cost-plus one.
- An under-measured Bill of Quantities directly reduces the builder’s margin, since the contract price can’t be adjusted after signing without a formal variation.
- A lump sum priced against an incomplete design carries genuine risk, since anything left unresolved at pricing time still has to be built for the same fixed figure.
- Every scope change needs to go through a variation process, which can create friction with clients who expect a “fixed price” to mean no further costs at all.
- Site conditions that turn out worse than assumed sit with the builder unless a specific provisional sum was carved out for that risk.
Who uses lump sum contracts
Lump sum contracts are used across almost every segment of Australian construction once a design is finalised — from a standard new home build, where the display-home-style contract is a textbook lump sum, through to commercial fit-outs and mid-sized developments tendered against a complete set of documents.
They’re less suited to early-stage or poorly documented work, which is why a builder asked to commit to a lump sum on a job that isn’t yet fully designed will usually push back for more documentation first, or suggest a cost-plus or provisional arrangement for the undefined parts instead.
Lump sum contract vs cost plus vs schedule of rates
A lump sum contract is one of several ways to structure how a builder is paid, and it’s worth being clear on how it differs from the alternatives:
- Lump sum vs cost plus — a lump sum fixes one total price upfront; a cost-plus contract pays the builder’s actual documented cost plus an agreed fee, confirmed only once the job is complete.
- Lump sum vs schedule of rates — a schedule of rates agrees rates for items of work without firm quantities, used where the amount of work isn’t yet known; a lump sum fixes both the quantities and the total price.
- Lump sum vs guaranteed maximum price — a GMP keeps cost-plus transparency while capping the client’s exposure; a lump sum simply fixes the number without disclosing the cost detail behind it.
How My Trade Hub helps with lump sum pricing
A lump sum price is only as reliable as the measured quantities behind it, which is exactly where My Trade Hub’s estimation engine earns its keep — generating a structured, measured Bill of Quantities directly from your plans, so the figure you commit to actually reflects the scope you’ll be asked to build.
Every rate, allowance and contingency line stays fully editable, so you can build a defensible lump sum price with confidence and assemble it 60-75% faster than manual estimation — without leaving your margin exposed to a rushed or incomplete takeoff.
Frequently asked questions
What is a lump sum contract in construction?
A lump sum contract is an agreement in which the builder completes a clearly defined scope of work for one fixed total price, agreed before construction starts and unchanged regardless of the builder’s actual cost to deliver it.
Is a lump sum contract the same as a fixed-price contract?
Yes. “Lump sum contract” and “fixed-price contract” are used interchangeably in Australian construction to describe the same arrangement — one agreed total price for a defined scope of work.
Can a lump sum contract price change after signing?
Only through a formal variation process. If the client changes the scope, or a genuinely unforeseen condition is agreed as a cost, the price is adjusted and approved before the extra work proceeds — the original lump sum otherwise stays fixed.
Does a lump sum contract include allowances like provisional sums?
Yes. Most lump sum contracts still include provisional sums or prime cost items for the specific parts of the scope that aren’t yet fully designed or selected, while the rest of the price remains fixed.
Who carries the risk of a cost overrun on a lump sum contract?
The builder does. Because the total price is fixed, any cost that exceeds what was priced reduces the builder’s margin rather than being passed on to the client, which is why accurate measurement at tender stage matters so much.
Is a lump sum contract suitable for a renovation?
It can be, but only if the existing conditions and full scope are genuinely known before pricing. Renovations with unknown conditions behind walls or under floors are often better suited to a cost-plus arrangement, or a lump sum contract with specific provisional sums carved out for the uncertain items.
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