Fixed-Price Building Contracts: What’s Actually Included (and What Isn’t)

The phrase “fixed price” gets used loosely in the Australian residential construction industry. Almost every builder’s quote is called “fixed” on the cover page, but the schedule of provisional sums quietly turns the same $520,000 contract into $580,000 by handover. The industry knows this. Some builders trade on it. Others — including our team — go the other way and fix every line item pre-contract, with no provisional sums on site works or inclusions.

Here is what a genuinely fixed-price building contract should include in Western Sydney in 2026, where “fixed price” typically breaks down, and how to read a schedule to spot the difference before you sign.

What “fixed price” actually means at contract law

The standard HIA New Homes Fixed Price Contract (used across NSW residential construction) uses the term “fixed price” to describe the contract sum. But the contract sum is composed of many line items, and specific mechanisms can move the sum:

• Provisional Sum (PS): a line item where the cost is estimated at contract signing. If the actual cost exceeds the PS, the owner pays the difference (plus builder margin). If less, the owner gets the credit.
• Prime Cost item (PC): similar to PS but typically used for owner-selected fixtures (appliances, tapware) with a defined allowance.
• Variation: any change to the scope or specification after signing. Priced at the time of variation.
• Contingency: not standard in residential contracts; occasionally used for genuine unknowns.

A “fixed price” contract with 15-25 provisional sums for site works, driveway, tile allowance, appliance allowance, landscape allowance, tapware allowance, air conditioning, piering, and so on is fixed in name only. Any of those provisionals can adjust the final price.

Where the provisional sums typically hide

The eight most common places to find provisional sums on a Western Sydney residential contract:

1. Piering (footings). Priced as “estimated 20 piers at $850 each = $17,000” with the actual count and cost to be determined at excavation. Real number often 24-32 piers = $20-$27k.
2. Driveway. “Driveway allowance $12,000” with finish and length to be confirmed. Real cost for a 25 m exposed aggregate driveway is $18-$24k.
3. Tile allowance. “Bathroom tile allowance $85/m² supply” — owner picks tiles at a specific tile shop, anything above allowance is charged, anything below is credit. Most owners find they want tiles at $110-$180/m².
4. Appliance allowance. “Kitchen appliance allowance $6,500” — most owners spec appliances at $8-$14k.
5. Tapware and mixer allowance. Similar to appliances, typically $2-$4k short of realistic spend.
6. Landscape allowance. “Landscape allowance $8,000 for turf and edging” — actual scope owners want is $15-$25k.
7. Air conditioning. “3-zone ducted A/C allowance” — actual scope for zoned 15 kW with 5-7 zones is 20-40% higher.
8. Rock or reactive-clay allowance. “$4,000 rock/adverse ground allowance” — actual excavation into reactive clay can be $8-$18k.

Add up eight provisional sums each $2-$10k under-allocated and the “fixed price” is quietly $30-$60k soft. That is common practice; it’s why the phrase gets a bad name.

What a genuinely fixed price should include

Our fixed-price contracts specify every line item to a real product and real quantity. Not “tile allowance $85/m²” but “600×600 mm Signorino Alba Bianco porcelain, laid in 43 m² across bathrooms 1 and 2, grout Mapei Ultracolor Plus Warm Grey, cost to owner $9,300 supply and installed”. Not “appliance allowance $6,500” but “Bosch Series 6 60 cm induction cooktop PIF645FB1E, Bosch Series 8 pyrolytic oven HBG634BB1, Bosch dishwasher SMS6HAI01A, Bosch canopy rangehood DWB77CM50A, cost to owner $8,850 supply and installed”.

Every fixture, every finish, every appliance is specified. The soil test is done pre-contract. The survey is done pre-contract. The structural engineer’s preliminary design is done pre-contract. Every quantity is real, priced, and fixed.

Our inclusions page shows the 200+ line items each with make, model and finish. Every one is fixed in the contract; there are no allowances.

How to read a schedule of provisional sums

Every HIA contract has a schedule of provisional sums (or notes their absence). When you receive a quote:

1. Turn to the schedule. Count the provisional sums.
2. Ask the builder to specify each one to a real product with a real cost.
3. Compare the sum of those specified numbers to the provisional totals. Typically the specified numbers are 20-40% higher.
4. Ask for the specified numbers to replace the provisional sums in the contract.

A builder who agrees to specify every provisional sum is running a genuine fixed-price shop. A builder who refuses — “we can’t lock the tile brand this early” — is running a shop where the price will move.

The four legitimate cases for a provisional sum

Not every provisional sum is a red flag. Four cases where they are legitimate:

1. Genuine unknowns. An asbestos survey on an existing dwelling before demolition is unavoidable — the extent isn’t known until testing. A provisional sum for asbestos removal is legitimate.
2. Owner-selected fixtures. If the owner explicitly wants to shop for tiles later, a PC item lets them do so without renegotiating the whole contract. But the allowance should be realistic, and any excess is on the owner.
3. Council imposed conditions post-approval. If a DA condition adds a specific requirement (e.g., an unusually tall boundary fence for privacy), a contract variation covers it. Sometimes an initial provisional sum anticipates likely conditions.
4. Statutory levy changes. Section 94 contributions or BASIX fee changes between contract and CC. Rare and small.

Variations — the other route the price can move

Even a genuinely fixed-price contract sees variations if the owner changes the scope. A common example: after the frame is up, the owner decides they want the kitchen island to be 200 mm wider. That’s a variation, priced at the time. A well-run project has 3-6 variations totalling $8-$18k across a $500k build; a poorly-scoped project has 25+ variations totalling $60-$120k.

The way to control this is thorough design and specification pre-contract. Every finish, every fixture, every dimension nailed down before signing. That’s what turns “fixed price” from a marketing phrase into a genuine commercial commitment.

Frequently asked questions

What is a provisional sum in a home building contract?
A line item where the cost is estimated at contract signing rather than fixed. If the actual cost exceeds the estimate, the owner pays the difference plus builder’s margin (typically 20-25%). Common on piering, driveways, tile allowances, and appliance allowances. Every provisional sum is a place the “fixed price” can move.

Can a fixed-price building contract have provisional sums?
Legally yes. The HIA fixed-price contract allows both fixed items and provisional sums. But if the schedule of provisional sums covers most of the finishes and site works, calling the total “fixed price” is misleading. A genuine fixed-price contract has few or no PS items, with specified products and quantities across all inclusions.

How can I tell if a builder’s fixed price will actually stay fixed?
Read the schedule of provisional sums (every HIA contract has one). Count the PS items and the total dollar value under PS. If the PS total is more than 15% of contract value, the price is soft. If there are zero PS items and every finish is specified with make, model and finish, the price is genuinely fixed.

Do I have to accept variations during construction?
Variations are only enforceable if you agree in writing under the contract’s variation clause. Any variation the builder proposes needs your written approval before it’s charged. If the builder requests a variation for something you didn’t ask for (say, higher-cost piering because the soil test came in worse than expected on a provisional sum), you can decline — but the underlying work still needs to be done, so the alternative is renegotiation of scope or cancellation.

What is the difference between a fixed price and a lump sum contract?
In residential construction they mean broadly the same thing — a contract sum agreed at signing. Both can be modified by provisional sums, PC items and variations. The key distinction is what proportion of the sum is truly fixed. Look at the schedule, not the label.

Should I negotiate specific inclusions into a fixed-price contract before signing?
Yes. Every finish and fixture you care about should be specified in the contract with make, model and finish. If you want a specific tile brand, name it. If you want a specific tapware brand, name it. If the builder’s default is an allowance, ask them to convert it to a specified product. That is what makes fixed price genuinely fixed.

Comparing quotes for a Western Sydney build? Get a fixed-fee quote from us — every line item specified to make, model and finish, zero provisional sums on site works or inclusions.