Construction contingency is the reserve you hold in a development budget for the cost overruns you know are coming but cannot itemise yet. It is a single percentage line, usually sized against your construction cost, and it sits between your builder’s price and your profit. Set it too thin and one wet-weather delay or one variation eats your margin. Set it too fat and you can talk yourself out of a deal that would have stacked up. This guide covers how much to allow at each stage, where the line sits in your feasibility, whether to calculate it on an ex-Goods and Services Tax (GST) or inc-Goods and Services Tax (GST) base, how lenders and quantity surveyors read it, and how it flows through to your margin.
The framing throughout is the developer’s: what the contingency does to what you can build, what it costs to carry, and what your return looks like once it is either spent or released. The rules of thumb are broadly national, because contingency is a budgeting convention rather than a regulation, so this guide leads with the Australian position and covers New Zealand (NZ) where the practice differs. Where a number depends on your deal, treat it as a starting point and price your own risk.
What is a construction contingency, and what is it actually for?
A construction contingency is an amount of money, usually expressed as a percentage of construction cost, held in the budget to cover the unknown or unresolved parts of a project. The Australian Institute of Architects describes a contingency sum as an allowance “included in the project budget to allow for the unknown or unresolved aspects of a design,” and notes that the early allowance can be as much as 25 to 30 per cent, falling to as little as 3 to 5 per cent of the contract price by the time construction starts.
The key word is “unknown.” A contingency covers the overruns that sit inside your defined scope but cannot be priced line by line yet: latent site conditions, a variation the design has not resolved, a subcontractor that prices higher than the estimate, a delay that adds preliminaries. It is not there to fund a scope change you choose to make later, and it is not a general slush fund. Most developers who run into trouble do so because they treated the contingency as spare money and spent it early, then met a genuine surprise with nothing left.
It also helps to be clear about what a contingency is not. It is not your builder’s margin, which is priced into the contract sum. It is not cost escalation, which is a forecast movement in prices over time and is better carried on its own line (more on that below). And it is not a provisional sum, which is a specific allowance for a defined item of work that has not been fully documented. Keeping these separate stops you from either double-counting risk or quietly leaving a gap.
How much construction contingency should you allow?
The honest answer is that it depends on how resolved your design is and how much risk the project carries, and the right number falls as both improve. A useful way to hold it is that contingency is the price of your remaining uncertainty, so it should shrink as the uncertainty does. The ranges below are widely used starting points, not rules.
Early feasibility: why 5 per cent is usually too thin
At the concept and early feasibility stage, a contingency in the 10 to 20 per cent range on construction cost is generally more realistic than the 5 per cent figure many first-time developers reach for. At this point the design is a sketch, the builder is not appointed, and the cost plan is an estimate built on rates rather than a tendered price. The Australian Institute of Architects puts the early allowance as high as 25 to 30 per cent for genuinely unresolved designs. You are not being pessimistic by carrying a double-digit contingency here; you are pricing the fact that almost nothing is fixed.
Carrying a fat contingency early has a second benefit: it makes your feasibility honest. A deal that only works on a 3 per cent contingency at concept stage is a deal that does not really work, and it is far cheaper to find that out now than after you have exchanged on the land.
As the design resolves: 5 to 10 per cent
Once you have a developed design, a firm scope and a competitive tender or a signed building contract, a contingency of 5 to 10 per cent on construction cost is the range most developers and financiers work with. This is also the band a quantity surveyor (QS) report for a construction loan will typically expect to see: a sensible allowance, sitting untouched, that tells the lender the project still has room to absorb a surprise. A detailed, well-tendered scope on a straightforward build might sit at the lower end; anything with unresolved elements sits higher.
At construction start and during the build: 3 to 5 per cent
By the time you have a fixed-price contract and construction is underway, the remaining contingency may reasonably fall to 3 to 5 per cent, because most of the pricing risk has been resolved into the contract sum. The Australian Institute of Architects notes this same drop to “as little as 3 to 5 per cent of the contract price” once construction starts. The caveat is the contract type: a fully documented lump-sum or guaranteed maximum price (GMP) contract transfers a large share of the build risk to the builder, so your own retained contingency can be lower. A construct-only contract on an incomplete design leaves more risk with you, so keep more back.
Adjusting for project type and risk
Two projects of the same value can justify very different contingencies. Some factors that tend to push the number up:
- Refurbishment, renovation and adaptive reuse, where you cannot see behind the walls until you open them up. These generally warrant a higher allowance than a new build on a clean site.
- Complex or constrained sites: heritage overlays, contamination, difficult access, deep basements, or a party-wall condition.
- Early or incomplete documentation, where the scope is still moving.
- A volatile trade market, where subcontractor pricing and availability are unpredictable.
A simple new build on a documented design in a stable market might carry 5 per cent. A heritage refurbishment with a half-resolved design might justify 15 per cent or more. The number is a judgement about your project, not a default you copy from the last one.
Where does contingency sit in your development budget?
Construction contingency sits inside your total development cost (TDC), immediately after construction costs and before profit. It is one of the roughly nine cost buckets that make up the full cost stack, and because it is a cost, every dollar of it comes straight out of your bottom line unless it goes unspent. That direct line to profit is why the number matters as much as it does.
Structurally, the cleanest way to model it is as a percentage applied to your construction cost total, shown as its own line so it is visible rather than buried. Enter it as a percentage on a basis you choose (ex-Goods and Services Tax (GST) or inc-Goods and Services Tax (GST) construction cost) and let the amount flow through into Total Development Cost (TDC) and the month-by-month development cashflow, spread on the same S-curve as construction spending. It is a single line to model in a tool like Feasly, and keeping it explicit rather than hidden inside the construction rate means you can flex it, report it to a lender, and see what it is doing to your return.
One modelling decision to make deliberately is whether any other cost lines are calculated on construction cost including or excluding contingency. Some fee lines, a project management fee, for instance, are sometimes struck as a percentage of construction cost plus contingency. If you set them that way, changing your contingency changes those fees too, so make sure the dependency is intended and not accidental.
Should you base contingency on ex-Goods and Services Tax (GST) or inc-Goods and Services Tax (GST) construction cost?
For a developer registered for Goods and Services Tax (GST) who is building to sell, the sensible base is generally the ex-Goods and Services Tax (GST) construction figure, because that is your real cost. When you build new residential premises or commercial stock for sale, you are generally making taxable supplies, so the Goods and Services Tax (GST) you pay on construction is generally recoverable from the Australian Taxation Office (ATO) as an input tax credit. The tax you pay the builder comes back to you, so the economic cost of the build, and the base your risk buffer should sit on, is the ex-Goods and Services Tax (GST) number.
Apply the same percentage to the inc-Goods and Services Tax (GST) construction figure and your reserve is roughly 10 per cent larger, because you are implicitly holding contingency against tax you expect to reclaim. On a straightforward build that is not a rounding error. Take a $4 million construction cost and a 7 per cent contingency: on the ex-Goods and Services Tax (GST) base the reserve is $280,000; on the inc-Goods and Services Tax (GST) base of $4.4 million it is $308,000. That $28,000 gap is not protecting you against anything real if the Goods and Services Tax (GST) is coming back to you anyway; it is just cost that makes the deal look worse than it is. The basis you pick should match the basis the rest of your feasibility runs on, which for most registered developers is ex-Goods and Services Tax (GST) across the board.
The contingency line itself carries no Goods and Services Tax (GST). It is a provision, not a payment for a supply, so no Goods and Services Tax (GST) event arises until the money is actually spent on works, and those works carry their own Goods and Services Tax (GST) and generate their own input tax credits when drawn. This is why a well-built model treats contingency as a non-taxable reserve rather than applying a notional 10 per cent to it.
Two situations change the picture, so flag them and get advice on your own deal. First, if you are building residential stock to hold and rent rather than sell, the residential rent is an input-taxed supply, which can deny or apportion your input tax credits on construction and shift the cost base you should reserve against. Second, if you are selling under the Goods and Services Tax (GST) margin scheme, the scheme changes the Goods and Services Tax (GST) you remit on the sale (one-eleventh of the margin rather than one-eleventh of the price) but does not change your input tax credit position on construction, so your contingency base is unaffected by that election. The margin scheme requires a written agreement with the buyer before settlement, and since 1 July 2018 buyers of new residential premises must withhold Goods and Services Tax (GST) at settlement and remit it directly to the Australian Taxation Office (ATO), at one-eleventh of the price or 7 per cent where the margin scheme applies.
Construction contingency versus the other buffers developers confuse it with
Contingency is one of several buffers in a development budget, and treating them as interchangeable is how projects either double-count risk or leave a hole. Each covers a different thing.
Design contingency covers cost growth as an incomplete design is resolved: the detail that gets added between concept and construction documentation. It is really an early-stage subset of your overall allowance, and it is why the total contingency is highest at concept and falls as the drawings firm up. If you carry a separate design contingency, make sure it is not also sitting inside your headline construction contingency.
Cost escalation is the forecast movement in construction prices between when you estimate and when you build. It is a market forecast, not a project-specific unknown, so it belongs on its own line rather than inside contingency. Burying escalation in contingency hides it, and on a project with a long lead time from feasibility to construction start, escalation can be the larger of the two.
Provisional sums and prime cost items are allowances for defined pieces of work that are not yet fully documented or selected, a provisional sum for a retaining wall, a prime cost allowance for tapware. They are specific, they sit in the contract sum, and they are adjusted up or down against the actual cost when the work is done. Contingency covers what you have not identified; provisional sums cover identified items you have not yet priced precisely.
The contract or superintendent’s contingency is the buffer a head contractor or superintendent may hold within the construction contract to administer variations. It is not the same as your developer’s contingency in the feasibility, and you should not rely on the builder’s buffer to cover your project risk. Hold your own.
How much should you add for cost escalation in 2026?
Escalation is running well above pre-pandemic norms and is forecast to stay there, so carrying a real escalation allowance separate from contingency is worth doing on any project with a lead time. Rider Levett Bucknall forecasts national construction cost growth of roughly 4 to 6 per cent for 2026, with cost escalation remaining elevated: around 4 per cent in Sydney and Melbourne, 5 per cent in Brisbane, 5.5 per cent on the Gold Coast, 5.3 per cent in Perth, 5.1 per cent in Adelaide and as high as 6 per cent in some regional markets. The Property Council has reported industry views that there is no relief until at least 2028, driven by skilled labour shortages, low productivity, subcontractor insolvencies and limited competition among tier-one contractors.
The practical point for your budget is timing. If your feasibility is priced today but you will not sign a building contract for twelve or eighteen months, an estimate that carries no escalation is understating your build by close to the annual rate for every year of delay. Model escalation as its own line, apply a rate appropriate to your market and program, and keep your contingency for the genuine unknowns. Two thin lines that each do their job beat one fat line that hides both.
How do lenders and quantity surveyors treat your contingency?
Lenders read your contingency as a signal of how much room the project has to absorb a shock, and they lean on a quantity surveyor (QS) to check it. On a development loan, the financier almost always requires an independent quantity surveyor (QS) report before releasing construction funds, and that report scrutinises two lines in particular: the contingency you have allowed, and the provisional sums in the budget. A quantity surveyor (QS) report that shows a sensible contingency sitting untouched tells the lender the project has genuine headroom. A contingency that is being drawn down early, before the build is far advanced, is a warning sign that the budget was tight to begin with.
The quantity surveyor (QS) does more than check the number once. On most facilities the same quantity surveyor (QS) inspects progress each month, certifies the work in place, and signs off each drawdown before the lender releases funds. If your contingency is being consumed faster than the build is progressing, that shows up in the monthly reports, and it can slow or complicate your drawdowns. Working with an experienced development finance broker helps here, because they know which lenders expect what level of contingency and how the quantity surveyor (QS) process runs.
A quick practical point: many lenders treat a materially untouched contingency at practical completion as available to help repay the facility, which is part of why they like to see it there. So a well-sized contingency is not dead money even if you never spend it. It reduces the lender’s risk while the build is on, and it can come back to you at the end.
How does contingency flow into your funding and loan-to-value ratio?
Because contingency is part of your cost base, it feeds directly into the ratios your lender uses to size debt. Your loan-to-value ratio (LVR) and your loan-to-cost ratio (LTC) both depend on where contingency sits, and different facilities are sized against different bases. A facility sized on Total Development Cost (TDC) includes contingency in the denominator. Some construction facilities are sized specifically against construction cost plus contingency, treating the two together as the build exposure the lender is funding.
This is worth matching in your model. Construction cost plus contingency is one of the bases a lender may size a facility against, alongside Total Development Cost (TDC), gross realisation value and others, so a feasibility model should let you pick the basis a particular lender actually uses. The point is that moving your contingency does not just change your cost total; it can change the debt you can draw and the equity you have to find. A larger contingency lifts your funded cost base, which can lift the facility on a cost-based measure, but it also lifts the total you need to fund. Model the two together rather than treating contingency as a number that only affects profit.
How should you size contingency for a bigger or riskier project?
On a large or high-uncertainty project, a flat percentage is a blunt tool, and a probabilistic estimate gives you a defensible number. Instead of picking 7 per cent because it feels about right, a probabilistic approach models the range of possible outcomes for each major cost element and risk, then reads the contingency off a confidence level. The Australian Government’s cost estimation guidance works in exactly these terms: it defines P50 and P90 as the costs with a 50 per cent and 90 per cent likelihood respectively of not being exceeded, with P50 being the median outcome. The gap between your base estimate and the P50 or P90 figure is, in effect, your contingency for that confidence level.
For Australian Government-funded projects over $25 million, probabilistic techniques are mandated to generate P50 and P90 outturn costs. Below that threshold the department accepts a deterministic approximation, where each major cost element and risk is assessed with an optimistic, most likely and pessimistic value and the spread is used to infer P50 and P90. Engineers Australia’s contingency guideline and state cost-estimating manuals such as the Queensland Department of Transport and Main Roads Project Cost Estimating Manual set out the same logic in more detail, and the Infrastructure NSW cost control framework applies the same confidence-level discipline to how contingency is governed and drawn down over a project.
Most private developments are nowhere near the $25 million public-project threshold, and a full Monte Carlo simulation is overkill for a townhouse project. The value for a developer is the mindset rather than the machinery: think in ranges, ask what a bad-but-plausible outcome costs, and size the reserve against the confidence level you actually need to hit your funding and profit tests, rather than defaulting to a round percentage. On a genuinely large or complex scheme, a probabilistic estimate is also easier to defend to a financier or an equity partner than a number you cannot explain.
How does contingency affect your feasibility and margin?
Every dollar of contingency you carry reduces your modelled profit until it is either spent or released, so the size of the line has a direct, measurable effect on your development margin. On a typical project where construction is the largest cost, moving your contingency from 5 per cent to 10 per cent can shift your margin on cost by a meaningful amount, and on a thin deal it can be the difference between a project that clears your hurdle and one that does not. That is the tension at the heart of the line: too little and you are exposed, too much and you may reject a workable deal.
The way to manage the tension is to test it rather than guess. Running your feasibility at several contingency levels shows you how sensitive the deal is to a construction blowout, which is often the single most useful stress test you can do. Flexing the cost base and reading the margin impact straight off shows how much of a cost overrun the project absorbs before the return falls below your target. A deal that stays above your hurdle at a 10 per cent overrun is a different risk proposition from one that fails at 4 per cent, and the contingency line is where you can see it.
There is also an upside worth modelling. A contingency that goes largely unspent flows back to profit at the end of the project, and on a well-run build it often does. That is not a reason to under-provision, but it is a reason to treat a healthy contingency as prudent rather than wasteful: it protects the downside during construction and, if the build goes well, much of it returns to you. The best practice most developers settle on is to size it honestly against the project’s real risk, hold it firmly during the build, and let the feasibility model show what happens to the return under each scenario.
Does construction contingency vary by state or territory?
No. Contingency is a budgeting convention, not a regulated figure, so there is no legislated contingency rate that differs between New South Wales, Victoria, Queensland, South Australia, Western Australia, Tasmania, the Australian Capital Territory or the Northern Territory. The percentage you carry is driven by your design stage, contract type and project risk, and those are national considerations rather than state ones. This is one of the few cost lines in a feasibility that does not need a state-by-state breakdown.
What does vary by market is the level of risk your contingency is covering. Construction cost pressure and insolvency risk are not uniform across the country. Rider Levett Bucknall notes Brisbane and the Gold Coast facing productivity and industrial-relations constraints ahead of the 2032 Olympics, and Melbourne contending with rising insolvencies and inflated baseline costs from mega-project overruns. So while the contingency convention is national, a prudent developer in a hotter or more disrupted market may sensibly sit at the higher end of the range for the same project. The rule is the same everywhere; the risk it prices is not.
How does contingency work for New Zealand developers?
The concept is identical for New Zealand (NZ) developers, with two differences worth knowing. First, the numbers sit slightly higher in common practice: New Zealand (NZ) feasibility guidance and bank quantity surveyor (QS) practice often reference contingencies of 10 to 15 per cent across the build, with more carried at early stages, reflecting the same design-resolution logic. As in Australia, a development financier will generally require an independent quantity surveyor (QS) report before releasing funds; the New Zealand Institute of Quantity Surveyors sets out the construction financing report framework that banks rely on, and firms provide bank quantity surveyor (QS) reports and drawdown certification the same way Australian lenders require.
Second, the ex-Goods and Services Tax (GST) versus inc-Goods and Services Tax (GST) basis point matters more, because New Zealand (NZ) Goods and Services Tax (GST) is 15 per cent rather than 10 per cent. A registered New Zealand (NZ) developer generally recovers the Goods and Services Tax (GST) on construction from Inland Revenue, so the sensible contingency base is again the exclusive figure, and applying the percentage to a Goods and Services Tax (GST)-inclusive construction number over-reserves by roughly 15 per cent rather than 10. One further risk driver to weigh in a New Zealand (NZ) contingency right now is planning uncertainty: with the Resource Management Act (RMA) being replaced and the Medium Density Residential Standards (MDRS) framework unsettled, timing and consenting risk on some projects is higher than usual, which can justify carrying a little more at the feasibility stage.
Common contingency mistakes that cost developers margin
The recurring errors are simple to name and expensive to make. Watch for these:
- Treating the contingency as spare money. Spending it early on scope you fancy, then meeting a real surprise with an empty reserve, is the classic way a budget unravels. Hold it for the unknowns.
- Carrying the same percentage from concept to completion. The number should fall as the design resolves and the contract firms up. A 10 per cent contingency that made sense at concept is generous once you hold a fixed-price contract.
- Burying escalation inside contingency. On a project with a long lead time, price movement can be larger than project risk. Carry escalation on its own line so you can see both.
- Applying the percentage to the wrong base. For a registered developer reclaiming the tax, calculate on the ex-Goods and Services Tax (GST) construction figure, not the inclusive one, or you will over-reserve and make the deal look worse than it is.
- Relying on the builder’s contingency. The buffer a contractor holds inside the contract sum is not your project contingency. Hold your own in the feasibility.
- Not testing it. A contingency you have not stress-tested is a guess. Run the feasibility at several levels and at a construction-blowout scenario so you know how much overrun the deal absorbs.
The bottom line
Construction contingency is the line that decides whether a development can take a hit and keep its margin. Size it against your real risk, not a habit: higher when the design is loose or the site is complex, lower once you hold a fixed-price contract, and honest at concept so a marginal deal shows itself before you commit. Calculate it on the ex-Goods and Services Tax (GST) construction base if you are a registered developer reclaiming the tax, keep it on its own visible line, and carry cost escalation separately. Then test it, because the contingency you never stress-tested is the one that lets you down. Held well, it protects the build and, more often than not, hands part of itself back to profit at the end.
This guide is general information for property developers and does not account for the specifics of your project. Contingency levels, Goods and Services Tax (GST) treatment and funding conditions depend on your circumstances, so confirm the position with your quantity surveyor (QS), accountant and financier before you rely on it.