The clause that costs people the most

Most building contracts land on the kitchen table looking complete. Prices are listed, finishes are described, and the total feels real. Then the build starts, and a second set of numbers appears. That second set almost always traces back to the same source: provisional sums, sometimes called PC items or prime cost allowances.

A fixed inclusion is exactly what it sounds like. The builder has priced a specific product, agreed to supply it at that price, and wears the risk if the cost moves. A provisional sum is an educated guess, written into the contract as a placeholder. If the actual cost comes in higher, which in 2026 it usually does, you pay the difference. The builder does not.

Why 2026 is a particularly risky year to leave things vague

South Australia has been recording strong approval numbers, with recent reporting pointing to an exceptionally active period for new home approvals in the state. A large greenfield project getting underway in Adelaide's south is one visible sign of that momentum. More homes approved and started means trades are busy. Busy trades charge more for their time, and that pressure flows directly into the line items most likely to be held as provisional sums: site earthworks, electrical fit-out, tiling, cabinetry and landscaping.

Victoria is seeing similar pressure in its outer growth corridors, where infrastructure is still catching up to lot releases. In both states, the gap between what a provisional sum says and what a subbcontractor actually quotes has widened. Builders set provisional sums conservatively when they write them. In a tight labour market, conservative is rarely enough.

Where the blowouts actually happen

The categories that cause the most grief are consistent across both states. Site costs, covering rock removal, soil treatment and service connections, are almost always provisional. A builder cannot know what is underground until excavation begins, which is fair. What is less fair is when flooring allowances, kitchen joinery, bathroom fixtures and electrical rough-in are also left as provisional sums, because those are things a builder absolutely can price in advance if pushed.

Lighting and electrical are a particular pressure point right now. The NCC 2025 seven-star NatHERS requirement applies to all new homes nationally, and meeting that standard often demands more insulation, double glazing and mechanical ventilation than older designs assumed. If your energy compliance solution has not been locked in before contract, the cost of meeting it can land as a variation rather than as part of your fixed price.

How the two states differ on builder accountability

Both states have mandatory insurance frameworks, but the details matter. In South Australia, your builder must hold a licence through the relevant state authority and maintain cover under the Home Building Compensation Fund. In Victoria, look for current VBA registration and confirm domestic building insurance is in place before any money changes hands. These requirements protect you if a builder becomes insolvent or abandons the job, but they do not protect you from a contract that is poorly specified. That protection only comes from the paperwork you negotiate before you sign.

What to do before you execute a contract

Go through the contract schedule line by line and mark every item that carries a provisional or prime cost allowance. Then ask the builder to convert as many of those as possible to fixed inclusions, with the actual product specified by name and model number. For any provisional sum that genuinely cannot be fixed yet, ask the builder to explain the methodology behind the figure and document that conversation in writing.

• Site costs: ask for a geotechnical report before signing if the block is anything other than flat, clear ground.
• Energy compliance: get your NatHERS assessment done early so the thermal envelope is locked in, not improvised during construction.
• Finishes: visit a display home or showroom and select your actual products before the contract is executed, so they can be listed as fixed inclusions.
• Electrical and lighting: request a fixed-price electrical quote from the builder's nominated subcontractor, not a provisional allowance.

What to watch

SA's approvals pipeline is full and trades are committed well forward. If you are contracting now for a build starting in mid-2027, build a contingency into your budget for provisional items moving higher, not lower. In Victoria, watch for any policy movement around first-home buyer incentives in the outer corridors, as increased demand there tends to tighten trade availability further and push up the costs sitting inside those provisional sum lines.