Example using abstract cost units
For an explicitly defined base of 200,000 cost units and a chosen 7.5% scenario, contingency is 15,000 and the total is 215,000. If a known omitted package costs another 20,000, add it to the base first: 220,000. At the same scenario percentage, contingency becomes 16,500 and total 236,500. These are arithmetic examples, not local construction rates or a recommended reserve.
Define what the percentage applies to
List the cost lines included in the base estimate and whether they already contain allowances. A contingency on direct work alone differs from one applied to a total including design, delivery and other services. Label the basis so another estimator can reproduce the number.
Choose the percentage through the project's estimating process and risk discussion. This guide supplies no universal safe percentage. Design maturity, unresolved conditions and the agreed procurement scope vary too much for one number to act as a guarantee.
Known work should have its own line
If the villa needs a kitchen and no kitchen quote has been entered, that is missing scope, not an unpredictable event. Add a clearly marked provisional amount or leave it visibly unpriced until a quote is obtained. Do not hide the kitchen inside contingency and then describe the base estimate as complete.
A provisional sum and a contingency also serve different purposes. The provisional line identifies work expected to be needed whose price or detail is unresolved. A separately tracked reserve can cover defined uncertainty around the estimate. Use your contract's terminology and conditions when recording these amounts.
Avoid duplicate allowances
Material waste, delivery increments, contractor markup and contingency are different calculation layers. A tile purchase rounded to full cartons may already include chosen cutting stock. Adding that same stock again as an unexplained general percentage would duplicate the quantity.
Likewise, a supplier's fixed quote may already include particular services or risks. Check what is included before adding another matching line. The goal is an estimate that exposes assumptions, not a series of percentages whose effects cannot be explained.
Track changes against the original basis
Keep the original estimate, the chosen reserve and approved changes in separate columns. When a risk turns into a priced item, record the amount and reason. This makes it possible to see whether the reserve is being used for genuine uncertainty or for scope that was omitted at the beginning.
When the project scope changes, revise the base and reconsider the percentage deliberately. A previously calculated reserve is not evidence that the new scope is funded. Retain dated quotes and drawing revisions so the comparison reflects the same work.
Do it in order
- List priced work and identify known unpriced scope separately.
- State the cost basis and the reason for the selected contingency percentage.
- Calculate the allowance and track approved changes without losing the original baseline.
Planning boundary
This worksheet organizes an estimate; it does not predict final cost or interpret contractual entitlement.
Is 10% always enough contingency?
No. The suitable allowance depends on the project's defined risks and estimate maturity. The tool calculates whichever justified percentage you enter.
Can contingency replace an unpriced bathroom package?
A known package should be a distinct priced or provisional line. Keeping it hidden inside a reserve conceals missing scope.