What ‘fixed price’ actually means
A fixed-price contract means the builder agrees to deliver the work described in the contract documents for an agreed price. It does not mean the amount you finally pay can never change. Variations you ask for, allowances that run over, and site conditions the contract excludes can all move it.
So the real question isn’t whether a contract is called fixed price. It’s how completely the documents describe the work, how realistic the allowances are, and how changes are handled. A detailed contract with honest allowances is worth more than a low headline number.
The scope: plans, engineering and a real specification
The price is only as fixed as the documents attached to it. Check that the contract includes the approved plans, the structural engineering and a detailed specification. The inclusions list should name brands, models and quantities for appliances, tapware, tiles, benchtops, flooring, windows and lighting — not just ‘quality fittings’ or ‘builder’s range’.
Anything not described is a future variation waiting to happen. If a feature matters to you, make sure it is written down in the contract or the specification.
Prime cost items and provisional sums
A prime cost (PC) item is an allowance for something not yet selected, such as an oven or a vanity. The NSW Government describes prime cost items as allowing flexibility for fixtures selected after signing, with the builder providing an estimated allowance. A provisional sum (PS) is an allowance for work that can’t yet be priced exactly, such as rock excavation.
If the real cost of a PC or PS item is higher than the allowance, you pay the difference, often with a margin. Ask for every PC and PS item to be listed with its amount, then compare those amounts with real prices for what you actually want. Low allowances make a quote look cheaper than it will be.
Site costs: fixed or provisional?
Excavation, retaining walls, rock, piers, drainage and service connections are where budgets most often blow out. Check whether each is a fixed amount or a provisional sum. With a contour survey and a soil test done before signing, most site costs can be priced firmly rather than left open.
Also check what is excluded — driveways, landscaping, fencing, letterbox, window coverings and air conditioning are commonly outside the base price.
Variations: always in writing, priced first
A variation is any change to the work after the contract is signed. The NSW Government states that variations must be in writing and signed by you and your contractor, and the cost implications should be explained before the work starts. Avoid any arrangement where changes are agreed on site verbally and priced at the end.
Good practice is a written variation that shows the change, the price, any effect on the build time, and both signatures, before the work is done.
Deposits, insurance and payment stages
For contracts over $20,000 NSW law sets several protections. The maximum deposit is 10 percent of the contract price. The builder cannot ask for payment until the Home Building Compensation Fund certificate has been provided to you. And the contract must include a progress payment schedule, with payments tied to completed stages of work rather than elapsed time.
A typical schedule follows the build: deposit, slab, frame, lock-up, fixing, practical completion. Each claim should match a stage that is actually finished.
Cooling-off, time and handover
For large jobs there is a cooling-off period of five clear business days after you receive a signed copy of the contract. Use it to read the whole contract and attachments, or to have a building lawyer review them.
Look for a stated build period, how extensions of time are claimed (for example for wet weather or variations) and what happens at practical completion: the final inspection, how the defects list is handled and when the final payment falls due.
Quick checklist
Builder’s name, licence and ABN match the public register. Plans, engineering and detailed specification attached. Every PC item and provisional sum listed with an amount. Site costs fixed where investigations allow. Exclusions listed. Variations must be written, priced and signed first. Deposit no more than 10 percent. HBCF certificate received before any payment. Progress payments tied to stages. Build period and extension rules stated.
Sources
NSW Government — Contracts for residential building work: https://www.nsw.gov.au/housing-and-construction/building-or-renovating-a-home/preparing/contracts
NSW Government — Checklist for contracts over $20,000: https://www.nsw.gov.au/housing-and-construction/building-or-renovating-a-home/preparing/contracts/checklist
icare — Home Building Compensation Fund, what we do: https://www.icare.nsw.gov.au/builders-and-homeowners/homeowners/what-we-do