How to spot a provisional sum trap in a builder's quote

Provisional sums look harmless on paper. But without understanding what they cover — and what they don't — a single line item can quietly turn a $50,000 allowance into a $500,000 problem.

If you've ever read a builder's quote, you've seen them: line items marked "PS" or "Provisional Sum" — placeholder amounts the builder uses when the scope, design or product selection isn't fully resolved at the time of quoting.

They look like budgeted costs. They aren't. They're estimates. And under most Australian residential building contracts, you pay the actual cost — whatever that turns out to be — plus the builder's margin on top.

What is a provisional sum?

A provisional sum (PS) is an allowance for work that hasn't been fully designed, specified or priced when the contract is signed. The builder inserts a number based on assumption, not measurement.

Common examples include:

• Site works (excavation, rock removal, retaining)
• Kitchens, bathrooms and joinery before selections are made
• Landscaping, driveways and external works
• Electrical and data fit-out beyond a basic allowance
• PC items — appliances, tapware, lighting

When the actual cost lands higher than the PS allowance, the difference is yours to pay — usually with the builder's margin added on the increase.

The $50,000 to $500,000 example

Here's a real pattern we see when reviewing residential and renovation quotes.

A homeowner receives a quote for a $1.4M extension. Buried in the cost summary is one line: "PS — Site works & excavation: $50,000." It looks reasonable. The homeowner signs.

Six weeks in, the builder issues a PC sum reconciliation and a series of variations. Rock is hit. Existing footings aren't where the survey suggested. A new retaining wall is required. Stormwater needs upgrading.

Final realised cost on that single line item: $500,000. Ten times the original allowance. None of it disputable — because the contract said "provisional", and provisional means we'll work it out later.

The lesson isn't that provisional sums are bad. They're a normal commercial tool. The lesson is that most homeowners and small builders don't read them critically before signing.

How to spot a high-risk PS

Not every provisional sum is dangerous. But several patterns reliably predict a blowout. When reviewing a quote, flag any line item that meets one or more of these:

1. The allowance looks low for the scope

A $50,000 site works PS on a sloping suburban block with no soil report attached is not an allowance — it's a placeholder. Compare the PS to recent comparable projects, not to the rest of the quote.

2. There's no documented basis for the number

Ask the builder, in writing, how the allowance was calculated. A real PS is built up from rates, quantities or sub-trade pricing. A guessed PS is just a number that makes the bottom line look acceptable.

3. The scope under the PS isn't defined

"Site works" can mean almost anything. A well-drafted PS lists exactly what's included — excavation depth assumptions, rock allowance, tip fees, retaining type — so any cost above that scope becomes a variation, not an open-ended bill.

4. The contract allows margin on the overrun

Most standard contracts let the builder add their margin (often 15–25%) on the difference between PS allowance and actual cost. A $450,000 overrun at 20% margin is another $90,000 — on top.

How to protect yourself before you sign

The good news: every one of these risks can be controlled before you commit. None of it is complicated. It just has to happen before signature, not after.

1. List every PS and PC sum in the quote on a single page. Total them. If they're more than ~10% of the contract value, the quote is not really a fixed price.
2. Request a written basis of estimate for each PS — a one-page breakdown of how the number was reached and what's assumed.
3. Convert what you can to fixed price. Anything that can be specified now (kitchen, bathroom selections, landscaping) should be priced now, not provisionally.
4. Cap the builder's margin on PS overruns — or remove it entirely on items that should have been priced properly.
5. Get an independent review of the quote and contract before you sign. A few hours of advice routinely identifies six-figure exposure.

The bottom line

Provisional sums aren't a scam. They're a normal part of construction contracting. But they shift risk from the builder to you — and most contracts make that risk almost invisible until the invoices start arriving.

A builder's quote isn't just a price. It's a risk document. Read it as one.