Prime Costs vs Provisional Sums

When you receive a quote or contract to build a new home, not every cost is fixed. 

Some figures are estimates, set aside for items you have not yet chosen or work that cannot be priced precisely until construction is underway. Two of these estimates appear in almost every building contract: prime cost items and provisional sums. 

Understanding what these allowances mean is essential, because they are the part of your contract most likely to move. A quote that looks competitive can quietly exceed your budget if its provisional sums and prime cost allowances have been set too low. This guide explains what a builder’s provisional sum is, how it differs from a prime cost, and, most importantly, how both affect the final cost of building your home. 

For transparent, fixed-price guidance on your project, speak with the custom home builders at Varcon Group today. 

What Is a Builder’s Provisional Sum? 

A provisional sum is an estimate of the cost of carrying out a specific part of the work, including both the materials and the labour involved, where the builder cannot give a definite price at the time the contract is signed, even after making all reasonable enquiries. 

Provisional sums typically apply to complete works rather than individual products. Common examples include: 

1. Site works and excavation, where ground conditions are not fully known 

2. Landscaping, driveways and external paving 

3. Retaining walls and rock removal

4. Connecting services where the scope is uncertain 

Because the actual cost is only confirmed once the work is done, the final figure can be higher or lower than the estimate. If it costs more, you generally pay the difference plus the builder’s margin; if it costs less, the saving is credited back to you. In Victoria, the way these sums are treated is governed by the Domestic Building Contracts Act 1995, which sets clear rules for how they must be described, calculated and charged. 

What Is a Prime Cost Item? 

A prime cost item is an allowance for the supply and delivery of a specific product, fixture or fitting that you have not yet selected, or whose price is not yet known, at the time the contract is signed. 

Prime cost items apply to the item itself rather than the labour to install it. Common examples include: 

1. Tapware, sinks and sanitaryware 

2. Ovens, cooktops and appliances 

3. Floor and wall tiles 

4. Door handles, hinges and other hardware 

5. Light fittings
   

The builder includes a reasonable allowance for a standard version of the item. When you make your final selection, the difference between the allowance and the actual price is adjusted through a variation. Choose a more expensive product and you pay the additional amount; choose something cheaper and you receive a credit. 

Prime Costs vs Provisional Sums: The Key Differences 

Prime cost items and provisional sums are frequently confused because both are estimates that can change. The simplest distinction is this: a prime cost applies to an item, while a provisional sum applies to complete works (including labour). The table below breaks down the differences. 

Quick rule of thumb: if it is a thing you will choose, such as a tap or an oven, it is usually a prime cost item. If it is a job that needs doing, such as excavation or landscaping, it is usually a provisional sum. 

A Worked Example: How the Numbers Move 

The clearest way to understand the budget impact is to follow the money. 

Prime cost example – the oven 

Your contract includes a prime cost allowance of $1,100 for a freestanding oven. During selections, you fall in love with a premium model priced at $2,300. The builder issues a variation for the $1,200 difference (plus any agreed margin), and your contract price rises accordingly. 

Provisional sum example – traffic management or site works 

Your contract includes a provisional sum of $1,600 for two days of traffic management at $800 per day. An unforeseen delay means five days are required. The builder issues a variation for the additional three days, or $2,400, again subject to the builder’s margin. 

In both cases the estimate moved upward, and in both cases your final cost increased. Multiply this across a dozen allowances in a single contract and you can see how a build can drift well beyond the headline quote if those allowances were set too low to begin with. 

How Provisional Sums and Prime Costs Affect Your Budget 

These allowances are the single biggest source of unexpected cost in residential building. The risk is rarely the allowances themselves; it is allowances that have been set unrealistically low to make a quote look more attractive. When that happens, the headline price is not the price you will actually pay. 

Key budget implications to understand: 

• Low allowances inflate cost exposure. A builder who allows $40 per square metre for tiles in a luxury home is almost guaranteeing a large variation later. The quote looks cheaper, but your real budget is higher. 

• Margin applies to the increase. When an allowance is exceeded, the builder is generally entitled to charge their margin on the additional amount, so an upgrade costs more than the price difference alone. 

• Allowances are excluded from the 15% rule. In Victoria, an owner can usually end a major domestic building contract if the price rises by more than 15%, but valid prime cost and provisional sum adjustments are ignored when calculating that rise. In other words, allowance blowouts do not give you an automatic exit. 

• They make quotes hard to compare. Two builders quoting the same home can show very different totals simply because one has set realistic allowances and the other has not. 

How To Protect Your Budget 

You cannot avoid allowances entirely, but you can manage your exposure. Before signing, take these steps: 

Scrutinise every allowance.

Ask what each prime cost and provisional sum is based on and whether it reflects the quality of home you are building. 

Compare allowances, not just totals.

When weighing up quotes, line up the allowances side by side. A lower total with skinny allowances is often the more expensive option. 

Make selections early where you can.

Every item you confirm before signing converts an estimate into a fixed cost, reducing uncertainty. 

Check the contract schedule.

Each allowance should be set out with a clear description, a breakdown of quantities and unit costs, and how any margin is calculated. 

Request evidence of actual costs.

You are entitled to see the invoices or receipts behind each allowance once the work is done or the item supplied. 

Hold a realistic contingency.

Even with sound allowances, set aside a buffer so that selection upgrades and minor variations do not derail your finances. 

Why This Matters for a High-End Home 

In a luxury build, the gap between a standard allowance and the finish you actually want is often substantial. Premium tapware, imported tiles, bespoke joinery and high-end appliances can far exceed the modest allowances found in a budget contract. If those allowances are not set with your standard of home in mind, the variations can add up quickly. 

The most reliable safeguard is a builder who prices realistically from the outset, sets allowances that reflect the quality you are aiming for, and is transparent about exactly where estimates sit in your contract. That honesty upfront is what keeps a project on budget, rather than a low quote that unravels during construction.  

A prime cost item is an allowance for an item you have not yet chosen, while a provisional sum is an estimate for complete works, including labour, that the builder cannot price precisely when the contract is signed. Both can move, and both directly affect your final cost, which is why realistic allowances matter far more than a low headline quote. 

Before you commit, read every allowance carefully, compare quotes on a like-for-like basis, and choose a builder who is open about how their estimates have been calculated. Doing so turns a confusing part of your contract into a clear, manageable element of your budget. 

If you are planning a bespoke home and want a transparent, accurately priced quote, speak with Varcon Group today. 

Frequently Asked Questions 

Is a provisional sum the same as a fixed price? 

No. A fixed price is locked in, whereas a provisional sum is an estimate that is reconciled against the actual cost once the work is complete. If the work costs more than the estimate you pay the difference, usually plus the builder’s margin; if it costs less, you receive a credit. 

Can a builder charge whatever they like once the work is done? 

No. In Victoria, the Domestic Building Contracts Act 1995 requires allowances to be calculated with reasonable care and skill, and the builder must provide invoices or receipts showing the actual cost. They cannot simply inflate the final figure beyond what the work genuinely cost. 

How much contingency should I allow for prime costs and provisional sums? 

It depends on how realistically the allowances were set and how high your finishes will be, but a contingency of around 10 to 15 per cent of the contract price is a sensible starting point for a custom home. The better the original allowances reflect your chosen quality, the less contingency you will need to draw on.