What a contingency is, and what it is not
A contingency is money set aside for work you know you will probably have to do but cannot yet describe. It is not a slush fund for upgrades, and it is not padding on individual estimates. The distinction is operational rather than semantic: a contingency has a release rule attached to it, and the rule is that it pays for conditions discovered in the building, never for a change of mind.
That single rule is what makes a contingency work. Renovation budgets rarely fail because of the discoveries — a rotten sill plate, a drain that runs the wrong way, a joist notched to half its depth by whoever fitted the bathroom in 1978. They fail because the discoveries and the upgrades came out of the same undifferentiated pot, so by the time the rotten sill appeared the buffer had already gone on better taps.
The flat ten per cent rule is popular because it is easy to remember, and it is wrong in both directions. On a new-build extension with a fully drawn scope, ten per cent is generous. On a century-old house where half the cost is behind plaster and the scope will be decided as walls open, ten per cent is not a buffer at all — it is a rounding error on the first surprise. This calculator replaces the round number with a sum of named risks, each of which you can see and argue with.
Add the result to your priced scope in the renovation budget calculator, which applies the contingency as a visible line on top of the four scope lines rather than burying it inside them.
The four risk factors, and why each earns its points
Every renovation starts at a 5% base. Even a perfectly documented job in a new house produces small surprises — a delivery short, a stud out of plane, a fitting that does not match the drawing — and 5% covers the ordinary friction of building work.
House age adds 0, 2, 4 or 6 points. Age is a proxy for how far the building departs from what current trades expect. Under twenty years, materials and methods match what a contractor works with daily. Between twenty and fifty, expect superseded fittings and a first generation of previous alterations. Between fifty and a hundred, framing dimensions, wiring and drainage all start to differ from modern practice. Past a hundred, add lath and plaster, cast iron, and finishes that may contain lead or asbestos and therefore have their own testing and disposal routes.
Concealed work adds one point for every ten per cent of the cost behind existing finishes. This is the most directly predictive factor, and the one people under-state. Ask which of your priced lines you could verify today with a torch and a tape. Repainting a room is nought per cent concealed. Re-plumbing a bathroom in a floor you have never opened is close to a hundred. Half of a typical kitchen refit is concealed, which is why the default lands at five points.
Scope certainty adds 0, 3 or 7 points. The test is simple: could a contractor build from what you have today without asking you a question? If yes, the scope is fixed. If the layout is settled but the finishes are not, expect changes and take three points. If the scope itself will be decided as the work proceeds — a common and perfectly legitimate way to renovate an old house — take seven, because you are choosing to buy information as you go.
Structural or systems work adds 4 points. Load paths, drainage, the electrical service and heating are the four areas where a discovery cannot be deferred or designed around. When a beam turns out to be undersized, you do not get to decide whether to fix it, and the cost is set by engineering rather than by taste.
The four increments are additive, so the framework runs from 5% at the lowest to 32% at the highest (5 + 6 + 10 + 7 + 4). It reproduces the familiar 10% exactly for a thirty-year-old house with 30% concealed work, a fixed scope and no systems involved — which is roughly the project the rule of thumb was invented for.
Worked example: a $40,000 kitchen in a 1965 house
You have priced a kitchen refit at $40,000. The house was built in 1965, so it is about 60 years old. Roughly half the cost — the plumbing, the wiring, the subfloor and the wall behind the units — sits behind finishes you cannot inspect. Your layout is settled but the appliances and worktop are not chosen, so the scope is likely to change. The job moves a drain and adds circuits, so it counts as systems work.
- Base. 5 points.
- Age. 60 years falls in the 50–99 band: 4 points.
- Concealed work. 50% × 0.10 = 5 points.
- Scope certainty. Likely changes: 3 points.
- Systems work. Yes: 4 points.
- Total. 5 + 4 + 5 + 3 + 4 = 21%.
- Fund. $40,000 × 0.21 = $8,400.
- Total to budget. $40,000 + $8,400 = $48,400.
- Against a flat 10%. $40,000 × 0.10 = $4,000, so this framework asks for $8,400 − $4,000 = $4,400 more.
Whether you accept that $4,400 is a real decision, and it should be made now rather than in week five. The alternatives are honest ones: reduce the concealed share by investigating before you commit — lifting a floorboard, running a drain camera, opening one wall — or reduce the scope-certainty points by finalising your selections before signing. Both actions genuinely lower the risk rather than merely lowering the number.
How to read and use the percentage
Below 10% means the project is well defined and the building is well understood. Take the low buffer, but check that you have earned it: a low concealed share is only true if you have actually looked, not if you have simply not thought about it.
Between 10% and 20% is where most renovation work sits. Hold the fund as cash or unused credit, not as scope you intend to add if things go well. A contingency you have already spent in your head is not a contingency.
Above 20% is a signal to buy information before you commit. At this level, the honest reading is that you do not yet know what the project is. Paying a few hundred dollars for investigation — opening up, testing, a structural opinion, a drain survey — routinely converts several thousand dollars of buffer into a firm price. That is the best return available anywhere in a renovation budget.
Release the fund deliberately and record every draw. A one-line reason against each release is enough: "rotten sill plate under window, $850". At the end of demolition, review what is left. If the fund is intact after the walls are open, you have genuinely reduced the remaining risk and can consider spending part of it on scope. If it is half gone, cut scope now, while cutting is still cheap.
Do not double-count the contractor's own risk allowance. A fixed-price contract already contains the contractor's margin for their risk, which is precisely why fixed prices exceed cost-plus estimates. Your contingency covers what the contract does not: variations forced by conditions, and the items your bid comparison showed were excluded or under-allowed.
The framework applied to typical projects
| Project | Age band | Concealed | Scope | Systems | Contingency |
|---|---|---|---|---|---|
| Repaint three rooms, 2010 house | 0 | 0% → 0 | Fixed → 0 | No → 0 | 5% |
| New flooring, 1995 house | 2 | 20% → 2 | Fixed → 0 | No → 0 | 9% |
| Bathroom refit, 1985 house | 2 | 60% → 6 | Likely → 3 | Yes → 4 | 20% |
| Kitchen refit, 1965 house | 4 | 50% → 5 | Likely → 3 | Yes → 4 | 21% |
| Basement finish, 1975 house | 2 | 40% → 4 | Likely → 3 | Yes → 4 | 18% |
| Whole-house refurbishment, 1905 | 6 | 80% → 8 | Exploratory → 7 | Yes → 4 | 30% |
| Extension on a 2005 house, fully drawn | 0 | 10% → 1 | Fixed → 0 | Yes → 4 | 10% |
Every figure in the last column is the sum of the four increments plus the 5% base, exactly as the calculator computes it.
Rules for running the fund
- Hold it in cash or undrawn credit, not in scope you plan to add. A buffer that exists only as an intention is not a buffer.
- Release only for conditions found in the building. Anything you choose — a better tile, an extra socket, a wider opening — is a scope increase and must be funded by cutting something else.
- Write one line of reason against every draw. The log is what lets you tell, at the end, whether the project was unlucky or badly defined.
- Review the balance at the end of demolition. This is the moment when unknowns become knowns, and the only point at which you can honestly re-plan.
- Never let the contingency fund a variation you have not priced. Get the number in writing before the work happens, not after.
- Do not restore the fund by cutting the last items on the schedule. Decoration, trim and snagging are what make the job look finished; cutting them converts a budget overrun into a permanent quality loss.
Where a contingency sits among the other buffers
Money is only one of the three things a renovation can run out of. The other two deserve their own allowances.
Time. Schedules overrun for the same reasons budgets do, and a delay carries costs that no cost contingency covers — extra container hire, extra weeks of a family eating out, an extra month of loan interest. Build the programme with the project duration calculator, which includes the waiting days for cure and delivery that labour estimates ignore, and price the disruption with the downtime cost calculator.
Scope. The most valuable contingency of all is a list, written before work starts, of what you will drop if the fund runs low. Deciding in advance that the utility-room cabinetry comes out before the kitchen worktop turns a crisis into an administrative step. Contractors call this a de-scoping list, and homeowners almost never make one.
Professional context. Cost engineers size contingency formally, by assigning probability distributions to individual risks and simulating the total — the method behind AACE International's guidance on contingency estimating. That machinery is overkill for a $40,000 kitchen, but the principle behind it is exactly the one used here: a contingency should be the sum of identified risks, not a percentage chosen because it sounds prudent. If you disagree with any increment on this page, change it and write down why. A buffer you can explain is worth more than one you cannot.
Finally, if you are weighing doing part of the work yourself, note that DIY changes the risk profile in both directions: it removes the contractor's margin, but it raises scope-certainty risk because amateur discovery takes longer to price and resolve. The DIY versus contractor calculator prices the labour side of that trade; the contingency belongs on top of whichever route you choose.