Provisional Sums and Prime Cost Items Explained

If I could make every person building a home read one page before signing their contract, it would be this one. Provisional sums and prime cost items are the single most common way a "fixed price" build stops being fixed. Not because they are dishonest tools, they exist for good reasons, but because most owners do not understand the maths inside them until the invoices arrive.

The two kinds of allowance, in plain English

Both are educated guesses written into your contract in place of real prices.

A prime cost item (PC) is a product you have not picked yet. The oven, the taps, the tiles, the vanity. The builder cannot price what you have not chosen, so the contract carries an allowance for supplying it, and official NSW guidance describes it exactly that way: items that "cannot be costed exactly before the work begins".

A provisional sum (PS) is a whole piece of work that cannot be confidently priced on signing day. Excavation is the classic, because nobody knows exactly what is under your topsoil until the digger finds it. A provisional sum covers labour and materials for that chunk of work.

The maths that catches people

Here is the mechanism, using the treatment in the standard HIA NSW contract (Master Builders contracts work similarly). Say your contract carries a PC allowance of $10,000 for tiles, and the tiles you fall in love with cost $14,000.

You pay the $4,000 difference, plus the builder's margin on that difference. At the default 20% margin that is another $800, so the tiles add $4,800 to your contract price. Now multiply that pattern across the oven, the tapware, the excavation provisional sum and half a dozen other lines, and you can see how a build "blows out" without a single thing going wrong on site. Nothing failed. The guesses were just low.

Two details worth knowing inside that maths:

• Underruns are credited without a bonus. If the actual cost comes in below the allowance, the difference is credited back to you, but no margin comes with it. Overruns cost you margin, underruns refund none, so the asymmetry always leans the builder's way.
• The margin should apply to the excess only. Under the HIA approach, the builder's margin on an overrun applies to the amount above the allowance, not the whole cost of the item, and HIA itself flags margin-on-the-whole-cost as a common billing error. Check any allowance claim against this before paying it.

Why low allowances win tenders

Here is the uncomfortable dynamic. A builder who writes realistic allowances shows you a higher, honest number. A builder who writes skinny allowances shows you a lower number that cannot survive contact with your actual taste. Guess which quote looks better on comparison day.

I am not saying every low allowance is a trick. Sometimes it reflects a genuinely modest specification. But when one tender sits well below the others, the allowance schedule is the first place I look, and it is usually where the difference lives. A cheap guess is not a cheap house. This is one of the core red flags I check when comparing builders.

How to shrink the allowances before you sign

The official advice from Building Commission NSW is the same as mine: choose your fittings and appliances before you sign, by brand name and model, so they can be priced rather than guessed. Every allowance you convert into a real, specified price is a line that can no longer blow out.

In practice:

1. Specify everything you can. Tiles, tapware, appliances, light fittings, floor finishes. On architect run projects this is exactly what the specification and schedules do, and it is a large part of why detailed documentation protects your budget better than any negotiation tactic.
2. Interrogate what remains. For each surviving allowance, ask: what does this figure assume? For a slab or excavation provisional sum, has a geotechnical report been done? A $2,000 excavation allowance on a sloping rocky Central Coast block is not an estimate, it is a placeholder for a future argument.
3. Ask the margin question directly. What percentage applies to overruns, and does it apply to the excess only? Get the answer in the contract, not the conversation.
4. Compare schedules, not totals. When quotes differ, line up the allowance schedules side by side. Same allowance lines, same assumed quantities? Now the totals mean something.
5. Keep a contingency anyway. Even honest allowances are still estimates. Quantity surveyors advising on residential work typically suggest holding around 10 to 15 per cent of the build cost in reserve, and allowances are a big part of why.

Where this sits in the bigger picture

Allowances are one chapter of a longer story about who carries risk in your contract. The rest of that story, deposits, progress payments, insurance and cooling off, is in my guide to the questions to ask before signing a building contract in NSW, and the whole journey from block to keys is mapped in before you build on the Central Coast.

And if you have a tender in front of you with an allowance schedule you cannot quite read, that is a normal state of affairs, not a personal failing. Reviewing those schedules against the drawings is part of what I do in a free site assessment. It is much cheaper before you sign than after.