You have a quote. This works out how much more you should be able to reach for, as a percentage band and a cash range, and shows you exactly why the number moved. Free, no sign-up, and the arithmetic runs in your browser.
Start with the quoted price of the building work. If you have several trade quotes, add them together.
Tick everything that is true. Each one adds or removes percentage points, and the panel on the right shows which.
Optional, and not part of the contingency: these are known costs that builders rarely include. Leave them at zero if they are already in the quote above.
No black box. Two lines of arithmetic and a table of adjustments you can argue with.
The whole model is this:
Contingency = Quoted building work × contingency rate
True budget = Quoted building work
+ Costs outside the quote
+ ContingencyEverything interesting is in the rate. The calculator starts from a rate for your project type, then adds or removes percentage points for how deep the work goes, how old the property is, and the specific risks you ticked. The total is held between 6% and 30%. Below 6% you are not carrying a contingency, you are hoping. Above 30% the honest reading is that the scope is not defined well enough to price yet, and another quote will tell you more than a bigger buffer will.
The result is then widened into a band: the rate plus and minus a quarter of itself, with a floor of 2 points and a ceiling of 6. A 17% job becomes 13% to 21%. That is deliberate. Anyone who hands you a single exact percentage for an unknown is selling you certainty that does not exist.
Worked through with the default numbers: a kitchen starts at 12, mid-range scope adds nothing, a 1970s property adds nothing, so the rate is 12%. The band is 12 ± 3, so 9% to 15%. On a €30,000 quote that is €2,700 to €4,500 of contingency, and a true budget of €32,700 to €34,500.
| One room, no wet work | 8% |
|---|---|
| Several rooms, no wet work | 9% |
| Kitchen | 12% |
| Exterior: roof, windows, cladding | 12% |
| Bathroom or wet room | 13% |
| Extension or addition | 14% |
| Loft or basement conversion | 17% |
| Whole house renovation | 18% |
| Cosmetic scope only | −3 |
|---|---|
| Mid-range scope | 0 |
| Structural or gut scope | +5 |
| Built 2000 or later | −2 |
| Built 1970 to 1999 | 0 |
| Built 1940 to 1969 | +2 |
| Built before 1940 | +4 |
| Moving or adding plumbing | +2 |
| Rewiring or new panel | +2 |
| Load-bearing wall altered | +3 |
| Permits not granted yet | +2 |
| Finishes not chosen yet | +3 |
| Listed or conservation | +3 |
| Starts more than 6 months out | +1 |
| Surveyed or opened up | −2 |
| Fixed price signed, full scope | −3 |
Five mechanisms, in rough order of how much money they move. None of them is bad luck.
Rot in a joist end, a cracked drain, aluminium wiring, a chimney breast someone removed without support, asbestos in an artex ceiling. This is the classic and it is real, but it is not the biggest number on most jobs. It dominates in one situation: an old property nobody has opened up. That is exactly the case where this calculator pushes the band into the twenties.
Spec creep is the quiet one. The tile you priced was €30 a square metre and the one you fell for in the showroom is €75. Multiply that by twenty decisions. Nothing went wrong, no one made a mistake, and the budget is 15% higher. Ticking “finishes not chosen yet” adds three points for precisely this, because an unchosen finish is an unpriced one.
“While you are there, could you also…” is the most expensive sentence in renovation. Said on site, agreed with a nod, priced at the end. Individually small, collectively the difference between a 10% overrun and a 25% one. The fix is procedural rather than financial: every change gets a number and a yes or no before it is built.
Once a permit is live, current code applies to what you touch and often to what is next to it. Smoke detection, insulation levels, fire doors, a service upgrade to carry the new load, structural calculations for an opening. These are not surprises to a builder who has seen the drawings, which is why an approved permit lowers your risk and a pending one raises it.
A quote is a snapshot. Materials, and the availability of the trades who fit them, move over a six to twelve month window, and long-lead items such as windows, structural steel and kitchen units are the ones that move most. If your start date is far out, ask how long the quote holds. That answer is worth more than a percentage point of buffer.
Four of these five are decisions, not accidents. They are made one at a time, on site, with no running total in front of anyone. The contingency is not really protection against bad luck. It is the budget for decisions you have not made yet, which is why knowing how much of it is left matters more than the size of it.
The same arithmetic, run end to end, with US and UK figures side by side. Quote values are illustrative of 2026 mid-market work; use your own.
New units, worktops and appliances, sink moving to the island, everything else staying where it is. Finishes not finalised. Kitchen 12, mid-range scope 0, built 1970–1999 0, moving plumbing +2, finishes not chosen +3 → 17%, banded to 13% to 21%.
| Line | United States | United Kingdom |
|---|---|---|
| Quoted building work | $45,000 | £18,000 |
| Contingency at 13% | $5,850 | £2,340 |
| Contingency at 21% | $9,450 | £3,780 |
| Appliances, bought separately | $3,000 | £1,200 |
| Design and permit fees | $1,500 | £600 |
| True budget, low | $55,350 | £22,140 |
| True budget, high | $58,950 | £23,580 |
Full rewire, new heating and plumbing, one wall out, kitchen and two bathrooms, permit applied for but not granted. Whole house 18, structural scope +5, pre-1940 +4, plumbing +2, rewire +2, load-bearing wall +3, permit pending +2 = 36, held at the 30% ceiling, banded to 24% to 36%.
| Line | United States | United Kingdom |
|---|---|---|
| Quoted building work | $150,000 | £100,000 |
| Contingency at 24% | $36,000 | £24,000 |
| Contingency at 36% | $54,000 | £36,000 |
| Design, engineer and permit fees | $9,000 | £6,000 |
| Appliances and fittings | $12,000 | £8,000 |
| Moving out and storage | $6,000 | £4,000 |
| True budget, low | $213,000 | £142,000 |
| True budget, high | $231,000 | £154,000 |
Useful for sanity-checking a quote and for splitting a total budget before you have any quotes at all.
| Area | Typical share |
|---|---|
| Kitchen | 20–30% |
| Bathrooms, all of them | 15–20% |
| Structure and services: roof, wiring, plumbing, heating | 20–25% |
| Living rooms and bedrooms: finishes only | 10–15% |
| Windows and external doors | 5–10% |
| Exterior and landscaping | 5–10% |
| Design, engineering and permit fees | 5–8% |
| Element | Share |
|---|---|
| Cabinetry and hardware | 29% |
| Installation labour | 17% |
| Appliances and ventilation | 14% |
| Worktops | 10% |
| Flooring | 7% |
| Lighting | 5% |
| Walls and ceilings | 5% |
| Design fees | 4% |
| Doors and windows | 4% |
| Taps and plumbing | 4% |
| Everything else | 1% |
If your total budget is €50,000 and you tell the builder you have €50,000, you do not have a contingency. You have a budget that will be fully spent. A contingency only functions if the number the project is allowed to consume is smaller than the number in the account. Say the smaller one out loud, keep the difference to yourself, and do not let the two merge in month four.
A quote that comes in above budget invites the obvious move: cut the contingency and proceed. It is the worst available cut, because it removes the only line that protects every other line. Cut scope instead. A postponed bathroom is recoverable; a project that stops at 80% complete because the money ran out is not.
Ten percent of the kitchen units is not a contingency for the kitchen. The rate belongs on the full cost of the building work, including labour and every trade. It does not belong on appliances you have already bought or fees you have already been invoiced for — those are known costs, and this calculator keeps them in their own line for exactly that reason.
The buffer is most valuable at the end, when the remaining work is fixed, the discoveries are behind you and there is nothing left to trade against. Spending it early on an upgrade that felt affordable in week two is how projects that were on track finish over. A rough guide: by the time you are halfway through the programme, you want more than half the contingency still there.
This is the one that actually costs money. Most people set a contingency and never measure it again, so the first indication that it is gone arrives with a final invoice. Every approved change is a withdrawal from it. If you cannot say today what percentage of your buffer is still available, the number you calculated at the start was decoration.
For a straightforward renovation of a modern property, 10–15% of the quoted building work is the normal planning range. For work that opens up walls, floors or services, 15–20% is more realistic. For a gut renovation of a property built before 1940, plan on 20–30%. The percentage applies to the quoted building work, not to appliances, furniture or fees you have already priced separately.
Ten percent is enough when three things are true: the property is modern, nothing structural is being touched, and every finish and fixture is already chosen and priced. Break any one of those and 10% is thin. The most common way a 10% buffer disappears is not a disaster behind a wall; it is a series of upgrade decisions made once work is under way.
No. Design fees, structural engineer fees, permit and planning fees are known costs, so budget them as their own line items — that is what the “costs outside the quote” section of the calculator is for. A contingency exists for costs you cannot yet name. Folding priced fees into it makes the buffer look bigger than it is and hides the fact that you have already spent part of it.
You keep it. That is the point. A contingency is not a spending allowance held back for the end of the job, and treating what is left as an upgrade budget in the final month is how a project that was on track finishes over. Decide before you start what unspent contingency is for — savings, the next phase, the garden — and write it down while you are still being sensible.
Yes, but less of one. A fixed price is fixed against the scope described in the contract, not against reality. Anything discovered once walls are open, anything a building inspector requires, and anything you change your mind about is a variation priced outside the fixed sum. A signed fixed-price contract covering the whole scope lowers the band here by three points. It does not take it to zero, and anyone telling you it does has not read a variations clause.
Twenty to thirty percent of the quoted building work. A gut renovation removes the two things that make a budget predictable: finished surfaces that hide the condition of the structure, and existing services that can be left alone. If the property predates 1940 and no one has opened up the floors, the top of that range is the honest planning number.
No. Tax is a known, calculable cost. Check whether your quote is tax inclusive or tax exclusive before you do anything else, because a 20% VAT surprise looks exactly like a blown budget and is entirely avoidable. Work out the contingency on whichever basis your quotes use, and keep every quote on the same basis so you are comparing like with like.
There is no obligation to. The contingency is your reserve, not part of the contract sum, and describing it as available budget rarely helps you. What you should absolutely share is the scope. Hiding an intention to move a wall later, in order to get a lower quote now, is the reliable way to turn a buffer into an overrun.
In commercial cost planning the two are held separately. The construction contingency covers unknowns inside the agreed scope — ground conditions, hidden damage — and is usually controlled by the builder or the cost consultant. The owner contingency covers changes the client decides to make. As a homeowner you hold both in one pot, which is exactly why it drains without anyone noticing: discoveries and your own upgrade choices come out of the same money.
One pot for the whole project is simpler and mathematically kinder, because a room that comes in under estimate can absorb a room that does not. What is worth doing per room is tracking the variance, so you know which part of the job is eating the pot while there is still time to react.
No. A provisional sum, or PC sum, is a placeholder for work that has been identified but not yet priced — a kitchen supply allowance, say, or an unknown quantity of repointing. It will be replaced by a real cost, and that real cost is usually higher than the placeholder. Contingency is for work nobody has identified at all. Keep the two separate and expect provisional sums to move.
Nothing is uploaded, because there is nowhere to upload it to: the arithmetic is a few lines of JavaScript running in your browser. The only copy of your inputs lives in your own browser storage, so the page remembers them if you come back. Start over clears it.
Being straight about this: the percentage bands above are a planning heuristic, not the output of a study we ran. There is no public dataset of what homeowners actually spent against what they were quoted, because almost nobody records both numbers. What exists is the following, and the model is built to be consistent with it.
Last reviewed . This is general planning information, not professional cost advice, and it does not know your local market. A quantity surveyor or an opening-up survey beats any percentage.
This calculator sizes the buffer. It cannot tell you how much of it is left in month five — and that is the number that decides whether a project lands. The Home Renovation Budget Tracker is the spreadsheet for that part: a change-order log where each approved change revises the affected budget line automatically, a contingency meter that turns amber at 25% remaining, a quote comparison tab that ranks bids on true all-in cost rather than base price, and a payment register that flags a duplicated invoice reference. One .xlsx file, six tabs, no macros, €15 once. The calculator on this page stays free and complete either way.
See the budget tracker