What is the difference between prime cost and provisional sum?

What is the difference between prime cost and provisional sum

Sometimes a building contract can read like it’s in another language. With confusing concepts and vague terms, figuring out what you’re agreeing to can be harder than first thought.

Just two of these phrases are prime cost and provisional sum. Here are the differences between the two.

Prime Cost

A prime cost is defined as ‘an item that has not been selected, or whose value is not known, at the time a domestic building contract is entered into and for the cost of supply and delivery of which the builder must make a reasonable allowance in the contract’

What this essentially means is that these are items that may not be selected yet – usually fixtures and fittings such as handles, stovetops and tapware – as it’s too early in the process. In order to ensure a consistent cost, the builder will make allowances for these items without knowing the specific cost. This allowance will include any mark-up or profit from the builder, as well as sourcing and installation costs.

Provisional Sums

A provisional sum is defined as ‘an estimate of the cost of carrying out particular work (including the cost of supplying any materials needed for the work) under a domestic building contract for which a builder, after making all reasonable enquiries, cannot give a definite amount at the time the contract is entered into.’

This translates to works that the builder cannot accurately price at the time of contract signing, like landscaping. It still includes both the provision of materials and the labour costs.

Most simply, prime cost applies to items themselves, provisional sums refers to complete works.

The process

If you do have prime cost items or provisional sums in your contract, the Domestic Building Contracts Act 1995 states that they must be set out in a separate document that includes:

  • A comprehensive description of the item
  • A reasoning for the cost estimate, with a breakdown of quantities and unit costs
  • How a builder will determine any additional charges that may be incurred

These allowances must be carefully considered before being included – in fact, it is illegal to include an estimate that is less than a reasonable cost. It is the responsibility of the builder to ensure that the suggested figures are as accurate as possible at the time of the contract being drawn up. The builder must also ensure that enough detail is included when describing the estimates, allowing for greater transparency and limiting uncertainty.

Once construction is complete, a builder must be able to provide evidence of the costs of any prime cost items or provisional sum allowances with invoices or receipts. If the actual amount is lower than what was estimated, the price is deducted from the contract price. If it is higher that what was allowed for, the owner must not only pay the difference but generally, also pay the profit margin for the builder.

At the end of the day, knowing exactly what you’re paying for is always preferable. However, there are always going to be circumstances where decisions have not been made, so making a calculated estimate is the only way to ensure a clear cost. Should you encounter a prime cost item or provisional sum allowance, ensure the separate document contains as much information as possible, encouraging a smooth and seamless construction process.


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To help you with your building journey we’ve put together the 6 key reasons why you should knock down rebuild.

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Thank the team for delivering us a beautiful family space. We could not be happier with the result and the process. Our dealings with Ramsay Builders were more than professional: they cared about our project and went beyond expectations. We found them to be a very thorough, considerate and respectful team of builders, who were happy to troubleshoot any issues as they arose. Our knockdown and rebuild project consisted of a modern contemporary house with lots of glass windows and doors and skylights built to a detailed architectural design and specification. We started our project in the midst of the pandemic and there was quite a bit of uncertainty around, and we were very concerned at first, but so lucky to have Brett and his team there, like a guardian angel, escorting us through a thunderstorm. The decision to choose Ramsay during the tender process was an easy one – based primarily on Brett’s communication, professional knowledge, state-of-the-art project management measures and most important of all, enthusiasm for the build. Though there were a handful of unexpected council planning challenges that arose during the build, none were too big for Brett to resolve and with minimal project impact too. The close relationships that he has built with his various tradespeople are quite evident and reflected in the high level of craftsmanship that were delivered in our finished product. For a person that has never built before, there can be lots of questions and “hearsay”, but Brett bridged this gap seamlessly and dispelled doubts by putting in the time to clearly explain and articulate how each aspect or process of the build worked. From the outset I was impressed with their organisation, reliability and ease to deal with. When some minor adjustments had to be made along the way, Brett and his team had good solutions and were responsive to requests. They were always focused on achieving a balance between quality outcome and affordability. The building was hence done with care and precision. We are over the moon with the result. Everyone who sees it is full of admiration. Importantly, they were also able to keep within our budget and time frame, ensured that we could move into our house on time, which we really appreciated, especially during such a challenging time. I know different people may have different building experiences, but with Ramsay Builders, you will only have one: assured. We would heartily recommend Brett and his team to anyone thinking about a build.

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